Probably not. Developers are fickle and few like to do maintenance only work. So even if it's still getting security patches they are likely to be losing the institutional knowledge to continue that in future, not to mention keeping up with things like API changes for gmail integration.

Which Gmail APIs? Has there been an update to IMAP that I'm not aware of?

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.