Completely agree - have you encountered this before? The Gmail plus sign alias trick has been widely known for a long time and, to my knowledge, still works well today. It would be easy enough for websites to either block + in gmail addresses or instead grab the true email.
Gmail also have "googlemail.com" alias and you can split your username with dots since they dont count like "[email protected]" and "[email protected]" are the same thing,
Nothing of it solves privacy though.
Spammers know to just cut out the +whatever. It's a simple regex to keep those from even getting into a database.
Guess what? There are some dumb website or applications complaining that the email address is invalid.
Some sites that block "+" in email addresses are actually just doing it out of incompetence. My credit union, for example, will actually accept an address with a "+" in it, but nothing will work because some broken bit of web 1.0 plumbing along the way converted it to a space (it shows up that way on my profile page). I wouldn't be surprised to see " " on my printed bank statements.