Read it again - they're unambiguously saying that non-Microsoft email programs that rely on SMTP, POP and/or IMAP will "stop functioning when Microsoft chooses to disable these protocols".
That the linked help clarifies a little doesn't mean this is a flatly misleading letter.
Well Microsoft blocks the Thunderbird embedded browser, so you can't complete the Oauth2 login. I've been effectively locked out of my account since (I'm not going to use the webmail just for this account - set up a forwarding email and told everyone to use my new address).
specifically what is missing, is that email using basic auth won't disconnected OCT 1st because of the tons and tons of machine that do things like email faxes, and scans that do not support modern authentication.
This looks like whoever "enterprise services" is, they wrote a brief message that wasn't very accurate.
"We won't support clients that dont support microsoft supported practices" is a very short stone's throw away from "we won't support non-microsoft clients", as I've come to discover at my university, where I had to ask for permission to use an email client just for oauth to work.
Which is yet to be granted after 2 months (and if granted, will be granted site-wide ... i.e. unlikely unless there is a large demand for non-microsoft clients)