Just because BYU used it in the 70s or whenever doesn't make it not the Yale Y
Yale was founded 100+ years before Joseph Smith was born. Quick GIS suggests Yale was using the block Y on football jerseys at least by 1888. Who knows when they started using it as a logo before that.
But in the end, who cares if it is similar? No reason BYU can't use it.
Mizzou is kind of screwed logo-wise in that they can't really use a block M since Michigan already claimed that association, but BYU doesn't have the same issue with Yale since they aren't FBS and are on opposite ends of the country, anyway. Even less of an issue if we lean into royal rather than navy.