By know I learned that remmina and FreeRDP are both free (and presumably open) implementations. RDP is the SM-Witzlos "Remote Desktop Protocol (RDP) […] proprietary protocol developed by Micky$oft".
And now it gets funny:
Remote Desktop Protocol: Non-Microsoft implementations
So, SM creates the proprietary RDP, FreeRDP offers server implementations for Mac and Windows, Weston builds up on FreeRDP to implement an rdp server it terms "rdp-backend" - and finally, SM-Witless uses that very "rdp-backend" for its own Linux Subsystem.FreeRDP offers server implementations for macOS and Windows. On other systems including Linux, software packages may build upon FreeRDP to implement a complete server. Weston, the compositor in Wayland, uses FreeRDP to implement an rdp server it terms "rdp-backend". This server is in turn used by Microsoft to provide graphics support (WSLg) in its Windows Subsystem for Linux.
Full circle:
proprietary - RDP
free - FreeRDP
free - FreeRDP+Weston = rdp-backend
free - rdp-backend used by SM-Witzlos for its Linux Subsystem (which is again as un-free as SM-Witless can manage it).