One of the things that drives me nuts about RV is the plethora of Zextensions. And on top of those, manufacturers add on their own proprietary extensions. That is one of the selling points. But...
Interestingly, in the Olden Days, machines often had custom instructions. The pdp-1/D had a tad instruction for 2's complement addition (it was normally 1's complement machine), and there were also new pdp-1 instructions for timesharing.
For the IBM 1401, there were all sorts of add on 'features', e.g. the Multiply / Divide Feature (sped up * and / in HW), High-Low-Equal Compare feature, Advanced Programming (added Index Register, Subroutine Linkage), Move Record feature (allows right-to-left movement until hitting a Record Mark, useful for Tape), Expanded Print Edit Feature (float $ sign, automatic * insertion, etc), various memory sizes from 1,400 characters up to 16,000 characters. The point is that manufacturer-distributed software had to deal with having features (or not); and this was handled e.g. in the assembler by having a CTL card that listed (coded) the features, e.g. "CTL 31110" says this is a 12K machine, with Automatic Multiply / Divide, the High-Low-Equal Compare, the Move Record Feature, but NOT the Expanded Print Edit Feature -- that told the Macro Generator what it needed to know to properly expand macros for a specific HW configuration. (Certain Features did not require SW mods, e.g. the notorious "Print Overlap" feature that would speed up print operations by having a hardware buffer to hold the Print Line, so the CPU did not need to stall. The feature was notorious because the sub-rack of HW required to implement it connected into probably 75% of the machine's instruction decoder & execution units; when it failed, it was incredibly painful to find the fault(s)).
And, there have been Writeable Control Stores like, forever. It was a feature on the Burroughs B1700, where different control stores could be loaded on-the-fly depending if you were executing COBOL or FORTRAN or Pascal - the 'instruction set' would be optimized for running that particular language. The pdp-11 had some version of this, and CMU's custom C.MMP had something like this.
So, the desire for certain customers to have machines specifically honed to their use cases was normal. It has only been this brief period of homogenization of single-chip(ish) CPUs (8008, 8080, Z80, 8086... amd64 etc) that introduced new instructions in tranches.
The End-Users now get their custom instructions in other ways (e.g. PCIe and GPUs)