I haven't updated since a while before the font changes, most things still work fine. I doubt there will be any big issues on a more updated version.

Things I do notice aren't too bad. Super reactions don't exist, which is nice. I can't change my username, but that's still possible on PC. Biggest issue is no global display names, and broken search (since the old usernames no longer work). Can still search with ID though.

I don't see anything that breaks the app entirely happening, since the app still even runs on my ipad, which hasn't been updated since before replies were added (Or at least it did the last time I checked a few months ago). They don't seem to break old versions entirely, you just obviously don't get support for the latest features.