Shoutout to the legendary Deluxe Paint III. I worked with people who, 30 years after the release, still kept an old computer running just to be able to use Dpaint II and III. They said it was the best pixel editor ever made.
Fun fact, Deluxe Paint I was open sourced…
https://computerhistory.org/blog/electronic-arts-deluxepaint...
There is the open source VGA Paint 386 to get something similar: https://www.bttr-software.de/products/vp386/
Runs in DOSBox (i.e. runs everywhere and can be expected to keep working).
And of course also the open sourced A**desk Animator on various platforms (but I run that in DOSBox as well... great for doodling on the phone).
https://github.com/AnimatorPro