There is μClinux [1] although it is not clear to me how much alive is the project

I wish I could run DiscoBSD/RetroBSD [2] on an ESP32, I like the idea of running on a MCU something that was originally meant for a PDP/11 (2.11 BSD)

[1] https://en.wikipedia.org/wiki/%CE%9CClinux

[2] https://github.com/chettrick/discobsd