I’ve known that the “Play Solo” option for studio was deprecated a long time ago, or at least I thought it was. But it was still possible to use via its shortcut or using the quick access bar.
However both of these methods are gone now 
If anyones wondering why I still chose to use play solo it was because it was easier then playing the entire game and losing your studio window (by using the play buttons or whatever) and it only opened 1 new window with your own character as opposed to starting a test server with 2 new windows.
Oh well.
[Posted previously by NWSpacek [Merged topic for more info]