From looking at the diff, it seems to be in the same vein as the IRC, XMPP, Google Talk(!), and some weird network I've never heard of "Odnoklassniki" (https://en.wikipedia.org/wiki/Odnoklassniki)
Fair enough, I guess. I never realized Thunderbird already supported those. I always had HexChat, Pidgin, or Fractal for IRC/GTalk/Matrix at the ready instead.
On the contrary; Thunderbird's chat has built-in IRC support which you can use without installing something like ChatZilla [1][2].
It's not very good, though, including warts such as automatically attempting an SASL login with your credentials, even if you've never registered a services account on the IRC network [3].
It's been a chat client for such a long time that I think it makes sense to add Matrix support, which is a much more modern protocol than the others supported. I think we've all just forgot that it has chat features, since it only supported protocols like Google Talk, IRC, and XMPP.
Thunderbird also has support for Usenet out of the box. I think the product sees itself more as a complete communication program than just an email program, even if that is what most people use it for.
I use Thunderbird, and I appreciated past "baked-in" additions like PGP support, but, I kinda just want my email client to be an email client.