On 4th January Google Login stopped working inside FB Messenger browser
I think this aspect was not really discussed previously - Google Login will also stop working inside Facebook Messenger browser on 4th January https://developers.googleblog.com/2020/08/guidance-for-our-effort-to-block-less-secure-browser-and-apps.html
This move is not only anticompetitive against new possible browsers but is also an attack against Facebook, one of the Google rivals.