I could use the opposite : A TODO-app which has no synchronization capabilities so I don't need to worry about leakage of private data. Ideally it would use a very simple plaintext file format for its database so it is future proof, e.g. http://todotxt.org/ The TODO.txt format seems very nice but the Android apps listed on that site are either not fully developed or unmaintained, and one even looks like it's not open…
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.…
I had tried Markor already, and its TODO feature seems like a prototype unfortunately. It lacks crucial UI to use it properly and it's clearly visible that the TODO part is "bolt-on" only on top of a text editor. :|
With regards to Todo.txt[2]: I can't manage to find its source code, is it open source?