If you say something illegal in a chat with a cop in it, or say it in public, I don’t think there are Constitutional issues with the police using that as evidence. (If you didn’t say anything illegal, you have a valid defence.)
Actual examples? No. I don’t believe it happened.
Hypothetical examples? Co-ordinating gunning down ICE agents. If the chat stays on topic to “coordinat[ing] legal observers,” there shouldn’t be liability. The risk with open chats is they can go off topic if unmoderated.
I don’t know if anyone IS using such a database unlawfully - they might be checking the plate number against an Excel sheet they created based on other reports from people opposed to ICE - but if its a databse they shouldn’t be using in this way, if might be against the law.
But that's not an example of something that would be illegal to say in a chat. It would be an example of something that's illegal to do regardless of the chat.