I agree with the folks that have said “hi, gl” and “gg”, are essentially “hello” and “goodbye” in my mind. They don’t really mean anything, but it’s nice to acknowledge your opponent.
I personally, am happy to chat if someone asks questions or wants to interact (I had someone ask me to critique their strategy post-game the other day, which was sort of odd, but I was happy to do it). But, I’ll rarely start an interaction on chat, unless something funny or cool happens in the game.
On the topic of general civility, I haven’t experienced any change really since auto-match was implemented. I’ve only had maybe 2-3 negative interactions in all of my games (mocking my strategy or calling me insulting names). When I first started playing, I was actually shocked by how nice and civil everyone was. I will say, there was a thread on BGG that made me think it would be nice to have a feature where I could make my own personal “blacklist” of people who are actively rude or extremely, extremely slow, but that’s sort of an idle desire, not something I think the community really needs.