I mean things like polls, additional message reactions, self chat, and pinned chats (looking at new features from last year) not just front end stuff like message density, theme changes, and whatnot.

Many of these probably just extend the existing API and others may not break things just not work but if someone pins a chat, starts a poll, or sends a new reaction and you don't know it because your client isn't keeping as up to date with these sorts of things it can be annoying at best and problematic at worst. It gets especially bad if you consider meeting chats, it seems like there is an endless stream of new functionality happening there like stuff around breakout rooms.

From what i could tell about 2 years ago, if you stuck to the skype (for business) API you would get functionnal messaging and maybe calls, but anything beyond sending and recieving messages (basically anything that you couldn't find in a skype for business client) would break and break often.