You might not be aware of Genode[0].
0. https://genode.org/
Genode looks interesting. As far as I understand it uses the sel4 kernel? Is it really in development since 2008?
It doesn't necessarily, but it can. Genode/SculptOS is kind of a microkernel OS framework, and it can use seL4 as the kernel.
Here is a talk about that porting effort:
https://m.youtube.com/watch?v=N624i4X1UDw