We can find the rationale for deprecating register in defect report 809: Deprecation of the register keyword which says (emphasis mine):
The register keyword serves very little function, offering no more
than a hint that a note says is typically ignored. It should be
deprecated in this version of the standard, freeing the reserved name
up for use in a future standard, much like auto has been re-used this
time around for being similarly useless.
The removal of register for C++17 was approved in the Lenexa meeting but it is still reserved for future use.
The register keyword was deprecated in the 2011 C++ standard, as its
effect was already implicit in the language. It remains reserved for
future use by the standard, and is time to remove its vestigial
specification.
Because of the as-if rule the compiler only has to emulate the observable behavior of the program and therefore the optimizer can via the as-if rule choose to keep a variable in a register if it won’t effect observable behavior and presumably will in most cases make better choices since it usually has more information.
For reference also see role of “register” C keyword? from the gcc mailing list, one of the replies in the thread says:
I don’t think the “register” keyword ever affected register allocation
in gcc. For that you have to go back to compilers of the 1970s.The register keyword does still have a use, though, in a gcc
extension: gcc uses it in combination with asm to implement register
variables.