The parent commenter is incorrect. Yes, the JXL authors do want to try to use JXL as a raw format, but they are focusing on other priorities right now, so the specification for how JXL could be used as a raw format doesn't exist yet. But the format is flexible enough to allow for it, without backwards-incompatible changes.
Ah, yeah, the current JPEG XL is not enough for the full RAW coverage. (I was talking more about per-channel color spaces.) There is indeed a reserved extra channel type for specific filter layouts among others, but its specification doesn't exist yet.
For some, possibly most, needs of using raw having yuv444, 14+ (or 16+) bits of precision and lossless or near-lossless (with strong guarantees of maximum error on individual pixels) compression is supposedly acceptable. I base this interpretation mostly on digital photography discussions I have seen on the internet. I am not a photographer myself.
(12 bits of component already requires adapting to the current lighting and can supposedly slow down professional photography.)