GMail has been pushing for its custom XOAUTH2 authentication for a while, and password-based authentication can result in security warnings. Password-based authentication also doesn't support 2FA.

By the way - the IETF standardised an OAuth2 SASL mechanism (RFC 7628) a couple of years ago, no idea why GMail doesn't support it.

This one?

https://developers.google.com/gmail/api/

I'm asking which Gmail APIs would Thunderbird have to keep up with. As far as I know, it never used that API, hence it doesn't have to keep up with it.

Sorry, didn't understand you right.

Basically, the only thing that is kind-of Gmail specific is the auth support (https://bugzilla.mozilla.org/show_bug.cgi?id=849540). If Gmail sometimes in future switches from OAuth2 to some other protocol, the client will have to be updated.

I haven't used thunderbird in a while, have they not added an easy setup for people using the popular online mail servers yet?

It seems that they have. I just started using Thunderbird again after a number of years and the setup to connect to my gmail account has been simplified to confirming the settings on one page with three auto-filled fields.

You don't need an API for that, just a few hardcoded settings that get auto-filled based on the domain.