I don’t feel hassled, but I do want to stress that this is totally hypothetical. My context is that I’m curious, and maybe threads like this will spur someone to have a good idea

If by “de facto public” you mean “not secure” then yeah. I said “infiltrate” because someone would still have to either join the local chat, find an informer, or hack in. All three of those are totally doable, and would happen, but it’s more involved than looking up a twitter page. But yeah, if the cops want in they’ll get in. The next layer of defense is to have only documented people without warrants in the chat itself, and keep things legal, so if cops harass them “it could happen to anyone” and there’s a bigger risk of a scandal. Undocumented people would get word from chat through contacts who are in it, but would not be in it themselves.

For vetting and actions to take, I figure it goes like this: someone suspects possible agents, posts photos or video and says “I think these people are with ICE,” then the group discusses and chooses whether to act. If yes, chat members warn everyone in their contacts, and those people pass it on to their contacts, and so on. Anyone undocumented in the area steers clear. Maybe people also show up to protest the agents, blow their cover, and film any arrests. This might be impossible with plainclothes agents in unmarked vehicles. But maybe this would force agents to stop masking and show their faces to blend in, which would be a minor win.

I don’t know if avoiding ICE agents is a viable strategy, though. Also, even if avoiding agents does work, a lot of victims will probably be at work when they are approached, and might be unable or unwilling to evade, especially if there’s no proof that the people in plainclothes are really from ICE.