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

Yes, it supports seL4 among a range of kernels.

Note the binaries are not specific to the kernel, so anything built for Genode will work on Genode systems of compatible ISA irrespective of kernel.

I am surprised to hear 2008, I could swear they have been active far longer. Maybe I am conflating it with TUD:OS.

They are indeed quite active. Just see their backlog of release notes. They release 4 times a year, on the clock, and always document what they've done.