I'm not sure you should expect much development for a microkernel that has been proven to meet its specification and to be free of bugs. Microkernels are supposed to have a very small API, so there's not much development needed there unless it doesn't the specification or has other kinds of bugs, which is obviously not the case here.
By it's nature at this state (after the full proving effort is complete), any changes to the main/master branch are inherently "stable" and PRs are only accepted once they reach that state.