People have made free hardware designs (https://www.gnu.org/philosophy/free-hardware-designs.en.html) for pen plotters and written free control software, which is somewhat usable for black and white printing as long as you are okay with low accuracy and smudging and often having to change pens.
Paper feeding and inking mechanisms are not trivial to implement and still heavily restricted by patents despite every variant being worked out 50 years ago (conversely 3D printing mechanisms are much easier, as people only really expect one workpiece, or a few on a bed of a fixed size to be printed out).
Kyocera printers run GNU/Linux, including GPLv2, LGPLv2.1 and GPLv3 software (they made some effort to exclude GPLv3 software, even though GPLv2 still requires installation information, but there are GPLv3 files in the software they use that was compiled in) and if they didn't intentionally infringe copyright by refusing to provide corresponding source code and installation information, getting free software to run on those printers would be hard but possible.