As a matter of antitrust law they wouldn’t be allowed to exclude BYU. Especially if Notre Dame was allowed to break off AND stay independent.
As a matter of good sense there is no reason they would want to. BYU is a reasonably competitive program with good viewership and a loyal national fan base.
If the five conferences break off before the whole program withers on the vine BYU will be included. And it will be the best thing to happen to the program since LaVell was coaching.