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.