Which "spec"? Many specifications exist and not every MTA, mailbox, nor email client understands the latest and every implementation is guaranteed to have quirks, but here are the most common: RFCs 822, 2822, 4952, 5336, 6530. There is no the spec. Subaddressing isn't standardized, but there was a half-hearted RFC on that with too many extra features. How email user parts are interpreted is up to the MTA, mailbox, or email client. Plus subaddressing informally as a hack has existed since at least 2005 according to my rusty memory. It's, therefore, a de facto standard. Use it where possible to track down leaks of personal information, but it's likely to bite with some gotchas. I used to maintain a @{{name}}.name domain and email server where all emails went to a single catchall account without setup or subaddressing needed to disambiguate which source email correspondence was from.
> Dots are somewhat common as optional these days but not universal.
Periods are optional.
Edit: Woops yeah, meant pluses. Dots are somewhat common as optional these days but not universal.