Originally Private teams used to have the ability to be set as discoverable via the client as well as the TAC. This ability was removed BUT the Powershell command was left in place to still set this if required.
Example: Set-Team -GroupId <group ID number> -ShowInTeamsSearchAndSuggestions:$True
Apparently even THIS has been disabled now and the command will not accept a true value. Does anyone know of any way to make private teams discoverable? This is a HORRIBLE design choice and creating a ton of sprawl in our tenant since users have no way of knowing if there's an existing Team they might need to be a part of or would like to be a part of to request access to. I still can't fathom why they took this option away and it's maddening.