I'm gonna die on this hill, but that was no different 20-30 years ago. We used to get yelled at in C Section to sit down. You could hear a pin drop when we were ranked #4 and down 10 in the second half to #18 Wisconsin in 2001. Bill Self used to complain about how quiet our crowds were.
Robert goes to way more games so I can't deny his perspective. But I went to several games last year I thought were just as loud and raccous as ever (Ayo night against Maryland comes to mind; Trent's personal 12-0 run in the first half against Arizona; the Curbelo show against Purdue on MLK day).
And by all accounts the second half comeback against Northwestern last week was as defeating as ever.
My point: I don't think crowds are any different now than they were 20 years ago. I think we romanticize what once was and project our feelings about what is happening now onto our perception of what the fans are or are not doing.
We get up for the really big games, wait for the team to show us something against the middling teams, and sit on our hands against the cupcakes.