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.
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.
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.
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.