I’ve designed a couple of mechanical keyboard PCBs (printed circuit boards) that are open source. The design files (for KiCad), bills of materials, documentation, etc. are on the htpkbs GitHub .
Inspired by the utility of the Postage Board and the aesthetics of the plaid keyboard, the pin-check is a small PCB that uses all through-hole components and is intended to be used as a controller for hand-wired keyboard builds. There are some build photos here.
The orch is a mostly ortholinear 4-row (plus an arrow key cluster and function key column) keyboard PCB that should fit most GMK keycap sets. There are some build photos here.