Init Official Support?

I run a runit-based system. It seems systemd and openrc are officially supported:

Is k3s trying to be agnostic of init system? Are folks running it on runit successfully long-term?

You can run k3s without using any service run by an init provider.
In the worst case scenario, you can just convert the systemd service files to whatever runit would be using.

There is not systemd specific magic in k3s, according to my experience working with k3s. Using systemd just makes it more convenient to use and some of the work is taken off your shoulders that way.