Latest Aseprite is still available with free (as in beer) source code to compile, even if it is a bit heavy on the dependencies these days, including requiring that you install a special fork of Skia iirc. I paid for it to get the pre-compiled binaries for Windows, but on Linux and OSX I always compiled it myself anyway. On FreeBSD, that is my desktop OS of choice now, I use the ancient open source version of Aseprite since that is what is most convenient to install (from the port). Maybe I should try Libresprite instead.

For my programmer art I also use old (Autodesk) Animator (in DOSBox) a lot. It is small and runs anywhere. Perfect for doodling on my phone, with some configuration to add various on-screen buttons in DOSBox. Small enough (less than 1 MB) that the entire application plus all configuration and saved working files can go into every source code repository where I want to edit some pixel art. https://github.com/AnimatorPro

Also have VGA Paint 386 installed in DOSBox everywhere. Have not used it much, but it seems good (probably more interesting for those that want something closer to a Deluxe Paint clone). https://www.bttr-software.de/products/vp386/

Then there is https://orama-interactive.itch.io/pixelorama that is open source and seems to improve at a good pace. I just never took the time to look very close.

Going to have a look at Tilf as well, to see if it is not too much work to get it to run in FreeBSD. Not being an expert in drawing anything, it helps to have many tools and switch between, as all tend to have something they do better (or easier) than the other ones.