In my experience most SaaS apps do not filter this out and allow re-sign ups with sub-addresses.
Gmail has an additional behavior that dot character is ignored in local component of the address . multiple@gmail.com, mult.iple@gmail.com mult.ip.le@gmail.com all route to the same inbox as well.
[1] https://datatracker.ietf.org/doc/html/rfc5233 [2] Less common in work hosted ESPs but almost universally default enabled in public ESPs for consumers.
Two or three mails have been misplaced in a decade.
It would be feasible to change something like that without breaking security now.
Google can hardly start allowing/routing a new account for first.last@gmail.com when you were getting it for years even though your account is firstlast@gmail.com and sensitive communication like say from your bank would routed there.