> Gmail has trained people to believe a bunch of distinct (according to RFC) mailboxes actually aren't distinct
I'd be fairly surprised if your average user of gmail knew this: I know it and I use it in part because it lets me _distinguish_ different accounts on the same site. Second-guessing someone who's taking advantage of this feature is more likely to generate tech support requests than not.
Knowledge of the plus trick has gotten pretty widespread. Anecdotally, I know a lot of non-technical people that use a single +spam address to route to a spam folder.
Non-anecdotally, articles with large numbers of views/comments about the trick can be found with a quick Google search on non-techie sites like NYT/HuffPost/BusinessInsider/Buzzfeed/Pinterest/etc. Not that those are definitive, but I think knowledge of this is more widespread than you think.
I'd be fairly surprised if your average user of gmail knew this: I know it and I use it in part because it lets me _distinguish_ different accounts on the same site. Second-guessing someone who's taking advantage of this feature is more likely to generate tech support requests than not.