Eventually google will require OAUTH for bounce processing and may require it for outbound SMTP through gmail servers
As noted on stackexchange, Google sent out an email outlining plans to require OAUTH and discontinue support for just username/password.
The way I read it, this is mostly applying to POP/IMAP clients for reading mail, and they are hinting that someday they might require it for outbound SMTP, i.e. at Administer - System Settings - Outbound Mail you choose SMTP and have it set to use gmail servers. The relevant quote is
No change is required for scanners or other devices using simple mail transfer protocol (SMTP) or LSAs to send emails. If you replace your device, look for one that sends email using OAuth.
(LSA means less secure apps, which is google-speak for username/password)
They are targeting June 2020 - Feb 2021 for IMAP etc, but as noted above they have not specifically set a date for changes regarding sending outbound emails.
It occurs to me though that bounce processing and the email processor use POP/IMAP. For the email processor you can work around it by just forwarding it to a non-gmail address and polling that, but bounce processing might be harder depending on setup.