Comment by p_l

Very much not the only one.

For example, a lot of systems used VxWorks that would run paravirtualized Windows 9x for user interface - you normally booted into windows, and during boot a .VXD file would get loaded that was actually a VxWorks bootstrap that preempted Windows kernel components and continued running them as "userspace" task in a way that allowed other realtime tasks to work.

This architecture was used for example in KUKA robots KRC1 controllers.

Similar setups are used by "PC PLC" products for example from Siemens.