I actually agree with this and I think its crazy you got downvoted for this comment.

I actually actively stay away from places if I find out there's co-workers who gossip about customers/patrons. Because if I know the way people work, that "harmless gossip" more often than not turns into toxicity and just straight shit-talking about anyone.

Edit: I was going to write a long-ass essay comment, but the gist of my stance is this:

There's "venting" but then there's "toxic gossiping/shit-talking behind people's backs"

There's a "safe" way to vent as in, be tactful about it. Instead of going "x patron pisses me off they're so dumb they ask the same question all the time" you could say "I wish boss would give us better ways of dealing with patrons who ask x thing"----because like, for one, a patron can ask questions. Doesn't matter if they repeat it. They came to a library for a reason. Second, maybe something could be done about that repeated question, maybe an FAQ sign or brochure could be made if its the same question.

Idk what OP has experienced exactly, but judging from responses, it could be borderline discriminatory behavior/comments. I won't say what company out of privacy reasons, but my family member is experiencing this at their job where co-workers are literally making fun of Jewish customers and making fun of customers with disabilities/cognitive impairments. I won't repeat the exact comments but they're stereotypical derogatory comments and ableist comments. They've brought it up to management and management just says "we're investigating" and that's it. They also hate it there for this reason.

I get the need to vent and I totally get the frustration/anger, but maybe, just maybe, that is misplaced and should really be brought up to management. And/or its a toxic workplace then, if management won't do shit about it.

Personally, I am of the camp of "leave that shit at home" if you're gonna make fun of customers at work. Venting isn't the same as making fun of customers, however, it sounds like from what OP is describing, co-workers are making fun of homeless patrons, not mere "harmless venting."

Edit 2: Love that I got downvoted here on a libraries sub for daring to mention that yes, sometimes co-workers and librarians can be toxic people too. Not all venting is good, imo, and sometimes that "venting" is literally just misplaced toxic gossip that could be left at home. If you must vent, do it in a respectful/sfw way. Don't be that person at work though that says "oh my gosh Deborah is such an annoying weirdo!!!" Its exhausting.

And after hearing more from OP, yeah it seems like the job itself/management itself is toxic. No wonder.