Re: I've seen a definite uptick in these
"I've tried emailing site owners with a link to the RFC that defines valid characters for email addresses"
To be honest it might be more productive to search Stack Overflow for "email address validation" and reply with corrections to the (presumably several hundred) crap answers that don't permit a plus sign, coz that's where all those crap websites got their code from.
Even more productive still would be to post an answer saying "Please don't even try to validate an email address. You *will* get it wrong.", because not allowing plus signs is just one of the common mistakes. (Ask anyone with an apostrophe in their name, for example.)