We recently added the ability to transfer ownership to another user when deleting a user’s account from AAD from the M365 admin portal. If no entry is provided, the default will transfer to the manager. https://docs.microsoft.com/en-us/onedrive/retention-and-deletion
The manager or newly assigned owner will have the ability to sync that content or download and then move to a shared library or their own OneDrive.Roger Valand commented
I second this. Dropbox does have this implemented and this works very well. People leave all the time and it's not always clear to everyone who's the owner. I would appreciate a menu choice next to or incorporated into the share file option, for transferring ownership to another person.