If you had a publicly accessible server, you could SSH to it and vi todo.txt there. But I'm not sure this would be worth the effort of setting up...

I would probably put the file in Dropbox . Easy to read on the phone. Can probably edit it too.

I do this with text files and add them to Dropbox, then mark them as offline files for easy access in the Dropbox mobile app.

I have another todo list in my mac's notes 'app' which syncs to my phone.

However, for non-work stuff, I usually rely on pen-and-paper lists.

It works for me, I've experimented with a few apps but they've never stuck within my workflow.

I strongly recommend Simpletask (disclosure, I've sent a couple PRs). Not visually polished, but functionality is the best, bar none: Lua scripts allow extensive customization of your workflow. Free & open source. Only on F-Droid due to asinine Play Store policies.

That's the thing. I attempted to get into the org productivity zone a while ago but it fell apart the moment I needed mobile support and syncing. Unfortunately, the mobile org clients of the time didn't support syncing well enough to be viable for my use case - not to mention that all the customization I had painstakingly set up within emacs didn't transfer to the mobile clients either.

I suspect it's impractical to adapt the Emacs workflow to the task of syncing data across devices that is now the expected norm, and other services will have to rise up in its place. If all you have is a filesystem, up-to-date syncing across multiple devices is just a really hard problem (coming from someone who could never get Syncthing to work reliably).

However, I have heard of the remoteStorage protocol, which is supposed to be an open-source syncing protocol that replaces Dropbox and the like, but it's still in its infancy. Maybe adding support for it as a plugin to sync directly from Emacs, instead of using a "dumb" polling filesystem monitor, could become an option.

https://remotestorage.io

On Android, there's Markor. It supports todo.txt, and I use Syncthing to keep it synchronized with my various devices and computers.