GMail has supported the "+" alias since the service was announced, one would think there'd be no excuse to not support it everywhere at this point. My consipiracy-theory hypothesis is that many companies "know" that any address with a + in it is an alias and actively filter it out. Because they don't want an alias, they want your _real_ address.
I run my own mail server and use a "." as the alias character. Haven't seen a system reject a single one of these.
Supported since forever by Cyrus IMAP for routing into subfolders.
For some unknown reason at Pitt people were taught to finish write their email as username+@pitt.edu ca. 1995. While it supported that as the inbox, most people were unaware that you could put +foo and have it go to the folder foo if one existed. But the address without the plus also worked.
Because gmail allows periods, and many people think the period is required since that's how they signed up for it. Therefore many people consider a period an important part of an email address.
I run my own mail server and use a "." as the alias character. Haven't seen a system reject a single one of these.