Almost every provider (social media, banks, websites and spammers) also know this, and routinely remove anything after + in @gmail addresses.

Indian Retirement Fund website, NPS, government operated, will take your [email protected] at signup, no errors, but will not let you login saying, Email Not Found. You have to login with [email protected] to get in your account.

All the places I've given addresses like that dutifully e-mails to the right place, so I guess I must've gotten lucky. But if you use your own domain with Fastmail (which I'd recommend anyway, so you have an easy way of moving elsewhere if you need to - how I was able to migrate to Fastmail from Google relatively painlessly in the first place) you can also set up catchall addresses so you can use the whole local-part instead:

https://www.fastmail.help/hc/en-us/articles/1500000277942-Ca...

Fastmail supports both [email protected] and [email protected]. I use the latter precisely to avoid the problem of sites that don't play well with plus addresses.