Folding keyboards are the bane of my existence...
Amazon had a bunch of them, but neither do I favour them, nor did any Amazon sites but Amazon US have them to my knowledge. Shipping from US to EU costs a fortune, so obviously that wasn't an option.
At some point, I got a white label (generic "Avatto" brand) Donar Electronic/Leetop BT1505:
Notice the seamless fold in the middle. When unfolded, they turn into a (mostly) standard keyboard that's pretty serviceable for touch typing.
The problem was that Steam Deck OLED's FC66E-B WiFi/BT chip (or firmware) glitches out when the keyboard is connected, unless it is the first connection after pairing. The Bluetooth keyboard chip in the keyboard is CYW20730A2KFBG, and another keyboard that appears to use it is Keychron K8.
Well, I made a very obvious mistake in the meantime, and got what appears to be a white label (generic brand) HT-B023:
Do you see it yet? No? Well, notice the gap on the fold; it is the exact thing I made sure to avoid the first time around! It is as bad as it looks; touch typing is much harder, not only due to the fold, but also the unusual key sizes. I feel so stupid for not seeing it right away.
I have no idea about its internals, and there are no FCC compliance docs to infer the information from, but at least this one doesn't cause issues with the Deck...