Hypothetical situation: someone had an abusive alcoholic father, and discussion of drinking and alcohol brings up a lot of unwanted feelings, including anxiety, unpleasant memories resurfacing, etc.

You're suggesting that the person in that scenario should either suffer those feelings in silence or should just keep trying new groups until they find one that just coincidentally doesn't bring up those topics?

Or someone who was sexually assaulted should keep that to themselves, and if discussion of the topic comes up in the game and makes them uncomfortable they should just leave and go find a new group without telling anyone why?

Are you suggesting that you could just say "hey, these are things that bother me so I'd rather not be part of the game if these topics are going to come up", and either the DM can exclude those topics, or they can refuse and the player can go somewhere else? Because that's exactly what is being discussed.

Lay out your ground rules. If there's no way to reconcile, go elsewhere, but it saves people getting blindsided halfway through the game and then having to deal with it while surrounded by other people and potentially feeling very vulnerable.