That being said, a kernel is a huge undertaking. There is nothing else with the scope of Linux. And substantial parts of writing a kernel involve dealing with architecture nastiness, and, in the absence of formal architecture models, no language will help much here. The Linux x86 low level code is a terrifying mess, and it’s also the best and most capable implementation I’m aware of. (I’m obviously biased.) A language like Rust would help only a tiny bit.

(Something like NMI handling on x86 is fundamentally memory unsafe. If you get nested NMIs, your stack gets clobbered. Thanks AMD. Linux has code, mostly in assembly, that detects and recovers. Good luck writing it in any memory safe language.)

I once skimmed seL4, and I would not use its x86 code for serious work in its current form.