That smaller venue is actually still there, just with a different name: "Hulu Theater At Madison Square Garden".
No doubt it’s still there. I think after the Felt Forum it was the Paramount Theater.
Maybe there was another name change in there somewhere else and I have no doubt knowing Jim Dolan (the MSG and teams ass owner) the naming of it changes with who’ll pay the most for naming rights.
Sucks because it was the Felt Forum for a long time back in the day.