That proposal is for contiguous typed collections, not for byte buffers. The fact that its use case is not the same as ContiguousBytes was mentioned in the thread.
Byte buffers are far from a narrow purpose; it's impossible to do pretty much anything involving a disk, a network, compression, encryption, etc. without a byte buffer being involved at some level. This is fairly fundamental, and there is currently no "protocol-oriented" way to do it without involving the Foundation library.