Same here. The Family Calendar disappeared from my Windows 10 Calendar app and from Office 365 Outlook in the last few days. On all my Windows machines. It also had to be added again manually on the web and mobile.
So, looks like Microsoft have decided to cripple it, and make it that Windows 10 cannot natively show your family events. Useless. For me, the whole advantage of the calendar is that it was just one click away on my desktop.
Typical 2010+ arrogant behaviour from a software company; encourage you to use a feature, then kill it without warning. Leaving countless individuals to spend
their time looking and then moving data to somewhere else. Pathetic.