Oh my god, I've wanted this for so long - lost so much progress over an accidental back press. Thanks for this! (though, I don't think it needs to be on by default - having it on by default could teach new users not to go into their communities UI)
Edit: though I do agree with the other user who mentioned a "do not ask again" checkbox - I think that would mitigate any concerns. Regardless, I'm just happy we have a warning now!
