The PX5 RTOS is ULTRASMALL (< 1KB for minimal use), enabling its use in some of the most memory-constrained devices. It is one of the smallest RTOSes available, requiring less than 1KB of FLASH and 1KB of RAM on typical 32-bit microcontrollers. I have not found out how it is licensed.
The company also offers https://px5rtos.com/px5-net/, a TCP/IP implementation.

Found on https://www.theregister.com/2023/11/28/microsoft_opens_sources_threadx/