A small number of consumer keyboards, and lots of DIY designs, some with prebuilt offerings, can run QMK or other open source firmware.
I'm not a sufficient keyboard nerd, I just want a compact well-spaced portable keyboard with Bluetooth and multi-device support, possibly also rechargable.
The market is full of such devices at decent pricing, but you need to cope with various firmware bugs which you only find after purchase...
[1] https://zmk.dev/