The plan is to migrate the teensylc branch to a new SmartMatrix Library 4 repo, where it will be released in parallel with SmartMatrix Library 3, not replacing. If I used the existing SmartMatrix Library 3 repo for SmartMatrix Library 4, someone could upgrade their library using Arduino Library Manager, and then wonder why their existing sketches don’t compile (because of the MatrixHardware headers at a minimum, and possibly other changes).
You make a good point though about introducing major changes to the teensylc branch, as enough people are using it that it would probably cause confusion if I quietly introduced these MatrixHardware header changes to that branch.