Okay, can someone explain why is this a thing? It is a genuine question. I am not very familiar with OS designs and skimming through the landing page didn't help. Is it essentially a Linux kernel that runs within an existing instance of Linux? Why is it important/useful? Thanks.
The idea was to have something you would call microservice architecture today, just for OS functions. An ethernet driver server application would talk to an IP server, which would talk to a TCP server that distributes messages to the applications. The kernel would just isolate all those applications and enable communication between them. If the ethernet driver daemon crashed you would just restart it.
The communication overhead remained a problem for microkernels, despite L4's achievements in that area. The advantages were somewhat overstated. The complexity merely moved from the kernel into the applications. A security bug in the IP driver daemon can still compromise the whole system, just like a driver in a monolithic kernel.