My friend, who is a delivery person, found a green stop sign. It is in an apartment complex parking lot. He said the name of the complex is Legacy @ Valley Ranch in Irving, Texas
It must be a roadway that's not publicly maintained and therefore isn't subject to the typical guidelines of American street signs.
Found before here: https://www.aaroads.com/forum/index.php?topic=1404
And the link in that thread is 404.