Many thanks Christian for both the re-assurance that at least your family's OneDrives have now re-started offering proper local sync of mapped/shared directories from other users again.
And for the the simple way to work out your OneDrive's backend version via online Settings > Options menu options 😎
Following that method it looks like my main OneDrive changing to only online URL shared links is because the main shared folder I map is now on OneDrive v2 (with 'Restore your OneDrive' as last option inside OneDrive Web > Settings > Options left menu) whereas my main OneDrive is still v1 backend (with 'Search' as last option inside OneDrive Web > Settings > Options left menu)
As both OneDrives are part of the same OneDrive Family Plan I'll keep my fingers crossed my main account migrates soon 🤞
Meantime as a workaround I've created a 2nd Windows login for that other MS account on my PC, then Pinned as Quick Access in File Explorer that resultant other user's OneDrive folder in my main login 😶
But now I'll need to login to 2 accounts on my PC every day! So that 2nd account's OneDrive will keep up to date with whatever changes I make in that favourite profile's OneDrive. The only good news is that if you are logged into both windows accounts at the same time, then when you access a file of the 2nd account's OneDrive it will download it (if needed) as part of the open request from main account. So you don't need to pre-sync locally all the 2nd account's files, they will work via On Demand.
Obviously that's a pretty horrible workaround! So please Microsoft hurry up with this migrations 🙏