I use the user+company@gmail.com alias when I can but MANY websites don't support that as a valid email address. Even tech companies that should know better.
I don't like it either, but it's a reasonable choice for them to make.
However, if I'm offering a service and collecting email addresses as part of that service, my mail form could do additional validation (e.g. I might not want to collect email addresses from the UK (.co.uk) or from .edu or from people with 'john' in their email address.
I'm not violating an RFC with my email form, which may provide some benefit (e.g. stop fake email addresses, reject customers I don't care about) at the expense of losing/angering some other customers. That's a business question, not a standards issue.
(BTW - I've never restricted email addresses on a form in this way, but can empathise with those who do)