There were attempts to use L4 and several other micro kernels like Coyotos and Viengoos, but as far as I know all of them stalled and the Mach variant has been

There were attempts to use L4 and several other micro kernels like Coyotos and Viengoos, but as far as I know all of them stalled and the Mach variant has been the only one to be continued to present day.