In 2020 obviously Utah and BYU didn't play
In 2019 the BYU/Utah game was Utah's 5th highest rated game, behind their bowl game, Oregon, Colorado, Washington and with UCLA and USC barely behind.
For BYU it was the 3rd highest rated, behind their bowl game and USC and just ahead of Tennessee and Washington.
In 2018, the BYU/Utah game drew garbage ratings since it was late in a lost season for BYU. It was BYU's 5th highest rated, and Utah's lowest rated national game.