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If you google the question, you will find an abundance of sources discussing the alleged fact that the Apollo missions left bags of fecal matter on the moon. A conversation on reddit got me looking deeper. Someone claimed that the story of Apollo astronauts leaving bags of poop on the moon is an urban legend started by a misunderstanding.

At first glance, I was incredulous. There were lots of sources, even NASA employees and scientists, talking about the poop on the moon. However, I noticed that all of these sources were recent. None of them contemporary. When I started digging into what evidence was used for this claim, I found most people pointed to catalogs such as this, which reference emesis bags. However, emesis is vomit, and while it is conceivable these bags may have been used for defecation, that is not definitive. They were supposed to use fecal collection bags. More critically, an official source claims the emesis bags left were empty spares.

[As Russell mentions in the comments, these catalogs also have "defecation collection device" listed, but those may also be empty spares.]

We have had a related question here:

Did astronauts on the moon poop in front of each other?

In this question it is discussed that defecation on the moon may not have occurred. However, it lacks a definitive answer to this question.

Is there contemporary evidence for the claim that bags containing fecal matter were left on the moon? Or have any of the Apollo astronauts explicitly verified this claim since the Apollo program ended? If not, is it possible to indicate whether the claims are plausible or straight-up legend?

I believe used fecal bags from the LM were normally supposed to be transferred back to the CSM and returned with the crew to Earth for analysis.

In the Apollo 15 flight journal, in the annotation after 174:14:00, we have this:

[As Apollo 15 disappears behind the Moon, Al switches on the Gamma-ray Spectrometer, X-ray Spectrometer and the Alpha Particle Spectrometer. The crew transfer all required items from the LM aided by a list in the Flight Plan that includes notes on where each item should be stored. The list includes film magazines, bagged rock and soil samples, core tubes including the very long deep core, food, used urine and fecal bags, and one of the OPS packages from the surface.]

That annotation was added after the fact, but in the Apollo 16 flight journal, at 191:19:09, while preparing to close out the LM for jettison, CAPCOM Haise gives the crew a checklist of items to transfer to the CSM:

191 19 09 Haise: Okay. Another little block you might write out to the left there, label it "Transfer Items." And, maybe you've already done some of these, but this will take care of some of the ones we missed having you do yesterday. And they're the PTK(?) [PPK], the Flight Kit, the purse with the unused food, and the used fecal-urine bags, and lastly the DSEA.

This doesn't speak directly to the earlier flights, but given that the used bags on the CSM were retained for analysis from day one, I think this would have been the procedure for all the moon landings. It also doesn't even prove fecal bags were used in the LM on Apollo 16; Haise almost certainly meant "if any were used, transfer them, and please don't tell me about it".

I can't swear Aldrin didn't toss a used diaper onto the moon, but I am strongly leaning towards "Myth: Busted" here.

Since the earlier flights involved shorter lunar stays, it's less likely that fecal bags were used at all; having to do one's business in the cramped LM cabin next to your crewmate would not have been pleasant. Lomotil and willpower can get you through two days pretty easily.

Table 2 in SP-368 Biomedical Results of Apollo appears to enumerate all the collected fecal samples from the Apollo missions, but unfortunately doesn't give timestamps or origin locations for them; it is also thus inconclusive.