This is important feature. We know for example you cannot see each other between 919 and 920 build.
If many players stay in the world and update is applied, players after that moment cannot join worlds and see others or cannot even warn others about update...
That is not good.
It is because VRChat has nothing to warn others there is new update out. There is no messaging system for it.
I think an update icon, in yellow color for example will solve the problem easy and they will see others again after restarting VRChat. You can put there like other icons (like voice, invite and request at same place.
It's no fun btw if you will miss friends for a while because they're still in old client for a while! What else can you do there? So please add the feature to make it better.
(I was also once in the world, still confusing why there is nobody in the world, jumping many times around, then finally i checked again, it was the update was just out after i started the client. BUH!)