I believe Markor[0] might satisfy your needs of having a TODO.txt editor that has no sync capabilities. I have to use in combination with Dropsync[1] to sync my notes.

Admittedly, I only use Markor for my plaintext notes and rely on Todo.txt[2] for my TODO.txt needs, which is still maintained and has syncing builtin.

[0]: https://gsantner.net/project/markor.html

[1]: https://play.google.com/store/apps/details?id=com.ttxapps.dr...

[2]: https://play.google.com/store/apps/details?id=net.c306.ttsup...