Could you please tell which RFC says this? According to RFC5321/5322 + can be used inside a local part but has no special meaning. Because a receiving MTA can interpret a local-part as it wants [1] some decided to treat a part after + as an extension, but it is not universal.
[1] RFC5321 "the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address"
"Consequently, and due to a long history of problems when intermediate hosts have attempted to optimize transport by modifying them, the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address."
Similar erroneous assumptions strip the dot from my gmail address in a mistaken belief that I will get it. All my non-dotted email goes direct to spam via a filter I wrote, because multiple-nines of spam strip the dot.
I've only had a single (government) organization refuse to leave the dot in my address, so I had to special case them.
> It is valid to remove +suffixes from email addresses (e.g. john+doe@example.com → john@example.com)
It is always valid to strip the +suffix. People won't like it, but the RFC says that part is always optional.