That must be for preserving the compatibility with the older software versions. The same happens in x86, where there are hundreds of obsolete instructions, whi

That must be for preserving the compatibility with the older software versions. The same happens in x86, where there are hundreds of obsolete instructions, which have been replaced by better instructions, but which are still supported to allow the execution of old programs. Both the new Arm SME instructions and the old Apple AMX instructions are executed by the same hardware matrix operation accelerator. Previously Arm has extended the Aarch64 ISA with the SVE instructions, in order to support the Fujitsu supercomputer. Then they were not satisfied with the original SVE and they have extended it into SVE2. I suppose that something similar has happened with SME. Apple must have negotiated with Arm the inclusion of matrix and vector operations implemented by a separate shared accelerator. The result was SME, which differs from the original AMX either because Apple has thought some improvements based on the experience with the first instruction set or because Arm has desired some changes from the Apple proposal.