I'm a bit ashamed to admit that I once owned a quite extensive list of "unintended" opcodes in then popular microprocessors like the 6502 or the Z80, but somehow lost it after having finished university and moved on to later technologies. 😭
So, at some time between the end of the 80s and now, my virtual cat or imagined dog must have eaten it. 😉
May be you can still find that stuff on the interwebs, hidden somewhere in between all that AI slop. Wikipedia might help you.
To give you a hint from ancient memories: The Z80 could architecturally access its registers in both 8 bit and 16 bit chunks - except for the IX and IY ones. Documented as index registers like the well-known HL one (i.e. concatenated H and L), they were portrayed as non-splittable. But they were as people found out, giving you more independently manipulatable registers for situations where you could take advantage of that capability. The IX/IY prefixes were functional pretty much everywhere. Later implementations of the Z80 architecture may have lost that undocumented capability.
