For ease of use and privacy, i think there should be a “Login with apple” button, using AppleID.
This is a login method i used for a long time, and i practically never use login with google, as it breaks practically any and every single one of my wishes of privacy.
I’ve not encountered any evidence that Apple provides any better privacy or security than Google does, at least regarding their accounts and SSO service. Then again, I’m not ITSEC, I just like to read random interesting reports and stuff from enthusiast groups! Regardless, I do think that integrating Apple’s SSO service would benefit a large number of users and should be worth it so long as it’s easy to bolt in. At least until the joint Microsoft-Google-Apple FIDO Alliance system gets up and running!
From the perspective of someone just using authentication services there’s not a lot of difference. A lot of google’s services and products have lesser privacy requirements, and this is from someone who kinda likes Google stuff and won’t touch Apple anymore. ^^
You probably mean the new FIDO2 stuff now, the FIDO alliance is already fairly fine and I use U2F keys all the time. 
Ah yeah! Hahaha FIDO2. The new one. XD
Yeah, i also thought not many of you would consider adding it, as it is not wildly used…