Brought to you by customMK, Bonsai C is a family of open source microcontroller boards intended for custom mechanical keyboards using STM32F ARM microcontrollers. Designed to be a drop-in-replacement for Proton C rev 2 (and thus Pro Micros as well), Bonsai C incorporates several design and manufactring improvements, including being "JLCPCB-ready".
Bonsai C3 features an STM32F303 microcontroller, and has the exact same electrical functionality as the (now unavailable) Proton C. More info on Bonsai C3 can be found here
Bonsai C4 Extended Edition features an STM32F411 microcontroller. It is designed to be a drop-in-repalcement for Proton C, making it also compatible with ProMicro-based designs. The board is available for purchase at customMK, and can be built through JLCPCB as well. In addition to the board being usable in DIY keyboard kits, Bonsai C4 can serve as a useful reference and starting point for keyboard designers planning to use STM32F411 becuase it incorporates the hardware necessary consistent USB DFU boot.
More info on Bonsai C4 Extended Edition can be found here, as well as a Bonsai C4 product guide here (work in progress)
Bonsai C4 Compact Edition is electrically identical to the Bonsai C4 Extended Edition, however, the Compact Edition physically matches the form factor of a ProMicro instead of Proton C. While lower part of the Extended Edition can be snapped off to make it the size of a ProMicro, doing this removes the FRAM and Flash memory chips. On the Compact Edition, the FRAM and Flash memory chips are instead mounted to the back side of the PCB. The board is available for purchase at customMK
More info on Bonsai C4 Compact Edition can be found here, as well as a Bonsai C4 product guide here (work in progress). There is also a JLCPCB-ready version of the compact edition, which meets hole size and trace/space requirements for JLCPCB, and uses parts availalble through JLCPCB.