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.
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.
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.
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.
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.
