Still super relevant in 2022 for the sites that allow it. Sadly, I just had a web programming class where the professor tried to tell me that + signs really are invalid.
I actually wanted to drop another usage I found and didn’t see in the comments, just in case someone else comes by. For websites where you might need multiple accounts (semi rare, but not unheard of) the + sign or full stop in an email address will not get recognized by the website as a duplicate email, so you can create accounts as needed without having to use up or create new email accounts.