From the linked site:
Microsoft 365 accounts set up as POP and IMAP will no longer connect to their mailboxes in email clients using Basic authentication. Outlook, Mac Mail, and other clients or programs that don’t support modern authentication (OAuth) for POP and IMAP won’t support these accounts.
K-9 Mail supports OAuth. See How do I add an Office365 account?
Also consider switching to an email provider that doesn’t make it incredibly hard to use a third-party email client via open protocols.