Sort of.

Thunderbird have taken over K9-Mail and are in the process or re-badging that as Thunderbird for Android. It won't be functionally different though.

So unless you are desperate for the Thunderbird name and logo you may as well use K9-Mail; I do and it's not at all bad.

Is there an iPhone version? Or could there be?

According to this they were supposed to be commencing work on an iOS version in 2023 but a release date was unknown. I can't find any more recent updates.

Is there an iPhone version? Or could there be?

https://connect.mozilla.org/t5/ideas/thunderbird-for-iphone-and-ipad/idc-p/55779/highlight/true#M32691

It's difficult to give a progress report or ETA yet, aside from re-confirming that we're committed to an iOS version of Thunderbird. We need to get a solid and polished Thunderbird for Android published first. In the meantime, we're currently hiring for a mobile manager, after which we'll start recruiting iOS developers. As things stand, I wouldn't expect to see any code until next year. But hopefully that flies by. We totally understand the demand for an iOS version!