For multilinguals (which is over half of the world's population), throwing in terms from another language or entirely switching to a different language mid-sentence during a conversation is entirely normal. I can't imagine it being that rare in personal notes.
Presumably, any number that prefer a terminal based workflow, or have the Vi key scheme burned into muscle memory. Or those that prefer an ultra-minimalist text editor for writing.
Just a decade ago, it was almost a meme that the Emacs and Vi factions fought over which editor is better. I imagine that those that truly bought into the Vi ecosystem/fandom use it in preference to other available options.
You're speaking to a veteran of said editor wars (vi won, naturally) and I still think the number of folks you describe is small these days. I imagine most have switched to a more GUI-friendly editor.