not every parameter in the observation search API endpoint is implemented in the observation search screen, and their default parameters are a little different, too (the screen filters for verifiable and non-spam by default, even if you don’t specify those parameters). i don’t think it could ever be assumed that every parameter available in the API was available on the screen.
if you really want to know if if a particular parameter is implemented, you could check the code in Github, or you could look at your browser’s network activity monitor to see if the screen is making a request in the background that includes the parameter, for example: