How to sync facebook contacts with ASUS ZenFone 5Z (Android 9)?
It looks like this hasn't been supported since 2016: https://m.facebook.com/help/community/question/?id=1028796497164750

I suppose it makes sense that if your social network data is your bread and butter, you probably don't want to be delivering it for free to just any old device out there.

It seems odd to me that this information was so hard to find, though.