Group creators can disable history sync if they wish, and when you join a group someone else created, you can see whether it’s enabled or not on the group details screen. I have it disabled in a bunch of my small, more intimate, groups for exactly this reason: so that nobody has to worry about someone else adding a less-trusted person to the group, who automatically gets a copy of history. But in larger groups that already include people who don’t know each other in real life, history sync makes threaded chat much more useful for new group members. And even in the absence of history sync, most people will already rationally treat these large groups as semi-public, knowing that any of the members you don’t already know and trust could easily leak what you’ve said to an audience outside the group.