Hmm, is the data on a personal device Microsoft's to delete?
I'd have thought it belongs to the owner of the device, it exists on the device only as a result of the owner using the app to create it, same as using, say, a text editor to create a text file.
In which case, surely it'd be illegal (at least in UK) for MS to delete it.
Of course, MS is welcome to disable the account at the other end, then it's up to the organization's IT to sort out the MS-created mess.
So glad I've almost finished migrating all my stuff off MS, goodbye O365, no more revenue for MS, hello Proton and Libre office.