Why in the world you want to remove a branch? I understand that you don't want to work on it but it does not hurt if it stays on the github repo, in fact, it could be useful for custom loaders (3ds_injector), the same thing goes for other entrypoints builds on the makefile.
Is not that I care about Luma's new way with Rosalina, after all, there is a fork with the legacy branch above and is not like Luma is the only working CFW/chainloader with b9s; we still have Cakes, ReiNand, the last build of Corbenik, BooCtr9 and CtrBootManager9, we don't need Luma's team shitty decitions, but don't be surprised if support on Luma (from users and other devs) decreaces.