Using a plus sign is subaddressing [1] and most ESPs[2] will route to the main address ( multiple@addre.es) . So you can use use multiple+email@adress.es, multiple+xyz@adress.es and both will route the email to you.
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.
