It is very likely that every extension will no longer be compatible by default for version 2.0. However, as with the release of 1.0 stable most active developers and developer teams will likely patch their extensions. For @FriendsOfFlarum - the foundation will again make a fund available so that all their extensions and the most used undermaintained extensions will be patched.
From experience I can say that it is very likely that a majority of communities will have migrated over to 2.0 within the first 3 months, some will have to decide to remove extensions before making that change, while others will have to either commission a patch, be patient waiting for one or crowdfund it.
As for a smooth upgrade; we always provide upgrade instructions to a new release and in case you run into any issues we will be available to guide you through fixing them.
username0136 I was very impatient, and ended-up installing it
I don't blame you, I would have done the same. But I hope that you didn't install 2.0. The most important reason is that no extension is compatible with it yet. So you would experience Flarum in its most vanilla way. There might also be bugs or even changes to how things work. So please do use 1.x for now.
