It's more Sendmail and Exim and others did it, and Gmail does it too - not the other way around;)
With qmail the "standard"[1] was username-alias@example.com, rather than username+alias@example.com. I rather prefer that - but then my (user)name(s) don't contain any hyphens.
Either way I think it looks better with system usernames - firstname.lastname@example.com was/is usually handled as a separate form of alias look-up anyway.
[1] http://www.lifewithqmail.org/lwq.html#dot-qmail-files See section "4.1.5. extension addresses"
[ed: and I recall using tmda as an anti-spam system - it inserts an encrypted tag in the alias portion, that can be stamped with an expiry date, and tied to a sender address (eg hmac(userkey, sender@example.com+01012018) => xyxyzzzzz - give sender@example.com the address user-xyxyzzzz@example.net - and mail to that address will be accepted from only sender@example.com until 1/1/2018.
http://tmda.sourceforge.net/cgi-bin/moin.cgi/AboutTmda
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