There's something more fundamental going on here: The concept of the ISA as a standard implemented by hardware, as opposed to the ISA being a document of what

There's something more fundamental going on here: The concept of the ISA as a standard implemented by hardware, as opposed to the ISA being a document of what a specific hardware design does, goes back to the IBM System/360 family of computers from 1964. That was the first range of computers, from large to (let's be honest) somewhat less large, that could all run most of the same software, modulo some specialized instructions and peripheral hardware like disk drives not present on the cheapest systems, at different levels of performance. As others have said, microcode is plenty old, but the idea of architectural families is one of the things which really gets us to where we are now.