One of the few people I know does this is Andre Staltz. He is active in Scuttlebut community, and is behind i.e. cycle-js fronted library/framework. I remember he uses a small portable keyboard to hack away on a library on a plane.

I tried to do this as well, and it is one of the reasons I bought Blackberry Key One with a physical keyboard. I do like to use the combination of termux running emacs to do some learning of programming languages, i.e. I did play around with Rust around this way.

More productive way to do actual programming that I tried, was to run jupyter-notebooks, as the notebook interface in the browser turned up to be a nicer interface to work in with a small touch-screen.

I wouldn't attempt any more serious work though, I would need to have the lager screen space and better key-board to write anything longer, or to read larger poritons of code. Most of my programming work consists of untangling and fixing interlinked pieces of code and I have to say, having more screen-space helps :P

On the other hand, maybe if you write APL, language famous for being succint, you might be able to productively work even on the small screen :-)