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I am curious if any groups prior to the 20th century (meaning that the peace sign is excluded) had a symbol to represent their revolt against an oppressor.

One could imagine branded slaves turning a fish (typical slave symbol in coastal cities) into a symbol which represents their control over their own bodies. Such is just an example.

The symbol must have been in use, probably as a seal, on clothing, or used as tattoos/branding. Symbols may also have been used by secret groups attempting to undermine despotic/dictatorial/monarchical civilizations.

Yes. A notable example is the Bonnet phrygien or Phrygian cap or the Liberty cap.

During the French Revolution it came to signify freedom and the
pursuit of liberty,...

Marianne, French symbol of liberty and reason, with a phrygian cap. Source: Marianne Républicaine

At the time of the revolution,

...l’antiquité romaine est très à la mode, et, quand la Révolution de
1789 éclate, c’est tout naturellement que Marianne, « Déesse » de la
Liberté, arborera le bonnet phrygien... porté dans l’antiquité par les esclaves nouvellement
affranchis.

Translation: ...Roman antiquity was very fashionable, and when the Revolution of 1789 broke out, it was quite natural that Marianne, "Goddess" of Liberty, should wear the Phrygian cap...worn in antiquity by newly freed slaves.

This symbol of the French revolution

was first seen publicly in May 1790, at a festival in Troyes adorning
a statue representing the nation, and at Lyon, on a lance carried by
the goddess Libertas. To this day the national emblem of France,
Marianne, is shown wearing a Phrygian cap. The caps were often
knitted by women known as Tricoteuse who sat beside the guillotine
during public executions in Paris in the French Revolution, supposedly
continuing to knit in between executions.

"...when the angry mob invaded the palace of King Louis XVI, they forced him to don a liberty cap, as shown in this French political cartoon of the 1790s." Text & image source.

In a comment below, TheHonRose mentions that

the Phrygian Cap was mistakenly used instead of the pileus

An article in Smithsonian Studies in American Art, Vol. 1, No. 2 (Autumn, 1987) by Yvonne Korshak throws some light on this. In The Liberty Cap as a Revolutionary Symbol in America and France she writes:

In the ancient world, the Roman pileus round or peaked, was the proper
cap to refer to Roman manumission. Over the centuries, however, the
Romans had represented a range of pileus types, and although the
peaked cap sometimes resembles the Phrygian, it does not duplicate
it, nor is one intended to represent the other. Furthermore, as the
cap of Eastern exotics, and thus of "foreigners," the Phrygian cap often appears on foreign captive and as a result came to be a visual
symbol of the prisoner. It appears that in the eighteenth century
the distinction between the cap of these foreign captives and the
pileus cap of the freed slave was blurred through the association of
both types of headgear with enslavement, a confusion abetted by a
plethora of antique cap types on illustrated monuments. Thus, the
Phrygian cap, while not originally a symbol of manumission, became, through confusion with the pileus, a symbol of liberty.

Sure. The hammer and sickle, representing "worker-peasant alliance" and later adopted by the Soviets as emblem of class struggle, dates from the 19th century. According to a numismatic site, its first appearance was on Chilean currency in 1894:

In the War of the Vendée, the Sacred Heart was the symbol of the rebels, which arouse from 1793 to 1800 against the French Revolution.

There were poor peasants on both sides of the French Revolution.

The Guerre en Vendée saw whole villages almost wiped out by the French Revolutionary Government.

Liste de massacres de la guerre de Vendée et de la Chouannerie

Du blason de la Vendée à son logo

The Vendée is a département in western France, located south of the Loire River and on the Atlantic coastline. The Vendée was the epicentre of the largest counter-revolutionary uprising of the French Revolution. In March 1793 provincial residents, who were never much interested with the Paris-based revolution or its ideas, took up arms against the National Convention. There werey many reasons for this uprising but chief among them were rising land taxes, the national government’s attacks on the church, the execution of Louis XVI, the expansion of the revolutionary war and the introduction of conscription. The people of the Vendée would pay a heavy price for their resistance. The government’s response was swift and triggered an internecine war in the region. The fight for control of the Vendée lasted three years and produced violence and mass killing that left the Parisian Terror in its wake. Sorokin suggests a conservative death toll of 58,000 but the real loss of life in the Vendée in 1793-96 may well be closer to 200,000.

Viewed retrospectively, the Vendée region had all the ingredients for counter-revolutionary sentiment. Located almost 300 miles from Paris, several days’ travel in the 1700s, it was distant and disconnected from events in the capital. The Vendée was almost entirely rural, with just a few towns and no major cities. The vast majority of Vendeans were relatively successful peasant farmers; their living conditions were better than those of their counterparts in northern France. The Vendée peasants were not as bitterly affected by the harvest failures and bitter winter of 1788-89. They enjoyed a comparatively better relationship with the First Estate; unlike elsewhere in France, the noblemen of the Vendée remained on their estates and did not act as absentee landlords. The citizens of the Vendée were also devoutly religious and dependent on their local parish and clergy. - THE VENDÉE UPRISING

Thus the Sacred Heart was the symbolic the Vendée rebels.

For those interested, here is the YouTube movie trailer video on this subject: War of the Vendee - Trailer

A simple leather shoe with strings as opposed to one with buckles. Peasents wore strings, rich people buckled shoes.

The Bundschuh movement (German: Bundschuh-Bewegung) refers to a series of localized peasant rebellions in southwestern Germany from 1493 to 1517. They were one of the causes of the German Peasants' War (1524–1526). The Bundschuh movement was not a movement in the proper sense, but a number of loosely linked local conspiracies and planned uprisings. It was so called because of the peasant shoe (Bundschuh) the peasants displayed on their flag. Under this flag, peasants and city dwellers had defeated the troops of the French count of Armagnac along the upper Rhine in 1439, 1443 and 1444.

Individual uprisings – seeking relief from oppressive taxes, arbitrary justice systems, high debts, costly ecclesiastic privileges, serfdom, prohibitions on hunting and fishing, and the like – occurred in 1476 in Niklashausen (Tauber valley), 1493 in Schlettstadt (now Sélestat)/Alsace (for the first time under the Bundschuh banner), 1502 in Bruchsal and Untergrombach, 1513 in Lehen (Breisgau), and 1517 along the upper Rhine. Each of these was defeated very quickly, and the leaders, such as Joß Fritz, were generally executed.

^{–– Solving the Mystery of the One-Booted Peasant: Hermeneutics Applied to the Seven of Wands 2018}

Bundschuh. A common word that strikes fear into every noble heart. Like the word Fire! in a crowded city. Almost as terrifying as Plague.

But now, it stands for the common man united in revolt against his overlords, be they secular nobility or ecclesiastical “Cloister Lords.” It symbolizes havoc, anarchy, famine, for without the peasants, how should we have food?

_{Wolfgang Capito's View – Life In The New Century (1500) –Bundschuh!}

^{(Three peasants talking – Albrecht Duerer ca 1497) (src 1, 2, 3)}

Showing realistic detail of the shoe in a low variant and a peasant carrying a sword that has seen battle – and was forbidden for him to own or have.

This kind of flag made it into the coat of arms for Bruchsal and is featured in a statue, before a bank(!) in Untergrombach, erected in 1989, commemorating Joß Fritz:

The red flag, which is prevalent on 20th century communist countries, has a long history of being linked to defiance. It was used by the Jacobins during the French Revolution, was adopted by Socialists during the Revolutions of 1848, and became a recognizable symbol of communism during the Paris Commune in 1871.