A Microsoft file hosting and synchronization service.
Hello,
As of today, Microsoft has not publicly shared any official details about the bonus storage.
I recommend bookmarking the FAQ page and checking back periodically for updates.
From my past experience with major Microsoft changes, such as Skype's retirement, Windows 10 ESU eligibility, and Teams personal leaving certain regions, I've noticed that Microsoft tends to update its FAQ pages over time as more information becomes available.
The Family storage change is still fairly new, so I expect Microsoft will provide more details as things progress.