I use this all the time, everywhere I can.

The worst example of "we know what are valid email addresses better than anyone" case was when adidas.com decided that they'd redesign their registration flow and in the process disabled logins with "invalid" email addresses when they previously had accepted registrations and logins with email addresses tagged with + for years. That was genius.

I wrote about this issue on my blog. Basically just using email aliases on a custom domain to enable per-provider filtering, and deletion of the alias if they ignore unsubscribe requests or get hacked. https://jonpurdy.com/2020/06/using-email-aliasing-to-detect-...

And most email scrapers / aggregators these days also ignore the + and whatever comes after.

Others strip it too