Markor

Install Termux and use your favorite TUI editor.

Simple and maintained:

https://f-droid.org/en/packages/org.billthefarmer.editor/

If you want to edit text files e.g. txt you can install Material Files from F-Droid and edit files there using built in text editor.

Does anyone know if there's a text editor with Material v3 design?

Acode... Not too simple, though

I use Squircle CE. It's not really just a simple text editor, but it's not complicated either. It should open pretty much any type of text document you throw at it.

It's not FOSS, and it's been a few years since I used it, but I used to rely on Jota text editor. It was a very straightforward one, no bells and whistles, but that was kinda what I wanted from it. It didn't get unmanageable when using very large files, and if I recall correctly I think it also handled both Unix and Windows line endings, which mattered to me.

Not sure if it's still good, though. I don't do nearly as much weird stuff with my phone as I used to.

Thanks for the suggestion, I am checking into them, now.