Does being maintained count as being actively developed? I think it should. As long as Thunderbird gets security patches, and maybe even the occasional bug fix, I'm happy with it.
I wish more pieces of software would get to the finished state and would slip into maintenance mode.
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.
That certainly sounds nice but it's not true. Or at the very least depends on how you define software. If software is built upon an evolving platform (such as the web) and has a self-supplying revenue stream if it needs one at all, then it can be done and will last.
Even for embedded software though, if something hasn't been looked at for years then the original developers probably don't remember it and may not still be around. If a client want's a feature or tweak then the institutional knowledge is gone and the best bet would be starting from scratch.
I wish more pieces of software would get to the finished state and would slip into maintenance mode.