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It is not part of any standard. + is a valid email address character and is not special in any way.

It's specified in RFC5233 [0], by "standard" I meant codified in RFC form, like everything else we treat as open internet "standards".

RFC 5223 uses + as an example. It doesn't even specify whether the detail part comes before or after the user part.

> On email systems that allow for 'subaddressing' or 'detailed addressing' (e.g., "ken+sieve@example.org"),

This rfc merely acknowledges that some systems do this. It doesn't say the must or even should.