Partitioning kernels have been used decades in aviation and other safety critical systems. I have developed software for integrity for example. It's commercial software and not proper open source, but the code is available for their their customers.
These system address the issue of separating different functions as well as possible. What they don't address the problems that widely used homogeneous systems face when a way to compromise the system is found.
I first encountered them made for aerospace for DO-178B. What's the oldest one you know like a separation kernel whose design details are publicly described? I like giving proper credit and knowing the history.
For me, we went from security kernels to Rushby's concept to separation kernels. Then, they did aerospace versioms first since safety requirements were already a subset of security requirements. Plus, aerospace companies would actually buy it. Hard to sell that stuff in non-regulated markets.
Separation kernel is efficiency thing. The oldest and most secure way to solve the same problem is separate computers and put data diodes between them if necessary. Need for integrated modular avionics was the driving force for the development. Aircraft have space and weight limits.
These system address the issue of separating different functions as well as possible. What they don't address the problems that widely used homogeneous systems face when a way to compromise the system is found.