Same here. I've been using a 'notes' file since I first got into Unix back in the late 80s. I've kept with it ever since. I still haven't found something I like better. So portable, easily searchable and easy to manipulate with standard unix tools, etc.
How is text not easy to modify on your phone? If anything this is a problem with phones being terrible computers and not text files. But I don't see how anything I said about how I do it on a desktop computer does not apply also to phones.
