Update: The full timeline for retirement of Basic Authentication in Exchange Online is now published in Basic Authentication Deprecation in Exchange Online – September 2022 Update.
For many years, ...
Updated Jul 09, 2024
Version 10.0
Blog Post
Again, without knowing more about your topology and versions, if I were faced myself with this challenge I would:
1. enable enough logging in my Exchange deoplyment on prem so I can lift for analysy pop3/imap/smtp (btw, ActiveSync is also a legacy protocol as far as I know)
2. Use something like https://techcommunity.microsoft.com/t5/Exchange-Team-Blog/Log-Parser-Studio-2-0-is-now-available/ba-p/593266 to anaylise them ( I would go for 1 month logs)
3. Would determine how many of my users are currently on non-adal versions of office, just in case.
I'm sure that all the brilliant minds roaming this forum might have different alternatives, this is just my view.
I used this method before myself to determine usage and then restrict the tenancy (fully disable legacy protocols on a per user basis)