I have probably thousands of hours clocked in C3 (desktop & chrome) while online so there is no way this "ready to use offline" notification hasn't popped up by now. I'm sure I've seen it once or twice.
However, without internet access...
If I try to launch C3 desktop version: Nothing happens. I get ghost nw.js processes.
If I try to launch C3 browser version from my desktop which I "installed" from Chrome: I get a blank white screen with a C3 logo so large I can only see the top half of it.
If I already have C3 open and lose my internet connection: Preview stops working.
If I already have C3 open and happen to be logged out: I cannot log back in and am locked out of paid features, preview, etc.
Is there, like, something else I need to do? Is this all just outdated information?
I vaguely recall one time I lost my internet connection while working, and C3 - after 5 minutes or so - just automatically switched to some offline version and I think it gave me a notification. I haven't gotten that since.