I think losing the ability to lurk in active communities of people I disagree with politically. I would never engage as it didn't seem right, especially down/upvoting, but It was always interesting to see things from the other perspective.
So places like r/tories, r/conservative, r/prolife, r/liberalgunowners - as a guy from the UK a lot of the subjects discussed in a few of those didn't affect me but I always prefer seeing things from both sides (even if I would never agree with them)
oh for sure, I was a member of the_donald and MGTOW (which was an incel sub). Absolutely didn't agree with either sub but it was fascinating—and especially scary—to see the communities evolve and introduce their own slang etc.