Sadly, most developers use some sort of ridiculous email validation that doesn't allow the plus sign.
Worse than that, I've encountered several sites and newsletters that allow the plus sign when you sign up, but their unsubscribe URL breaks (presumably due to some escaping/encoding mismatch), preventing you from unsubscribing.
I have seen sites like meetup.com that disallow the "+" sign for spam reason. Instead of having proper spam detection and filtering they just added arbitrary rules like that.
Worse than that, I've encountered several sites and newsletters that allow the plus sign when you sign up, but their unsubscribe URL breaks (presumably due to some escaping/encoding mismatch), preventing you from unsubscribing.