Meanwhile, I have spoken to MS re: the Family Calendar disappearing from the desktop client. They advised:
As I checked from my side, the issue is from Outlook.com server and our developers are acknowledged about this inconvenience problem that cause to our dedicated customers. Thus, they are investigating and working to fix this issue.
They do not have a timescale for the fix at this point.
I suggest going through the virtual agent (which won't help) to get to a real person to flag the issue - the more noise the greater the effort they put in.
I have also sent a tweet to the @Outlook team; I suggest everyone does the same:
https://twitter.com/simonjhudson/status/1301462713923764224