I remember the knowledge/idea of illegal opcodes for the C64 slowly making their way through the hacker/cracker community. Like secret knowledge being distilled back then via BBS
they were quite useful. I distinctly remember there were undocumented NOPs consuming a different number of cycles than the default one EA. These were fe used to pad raster line interrupt routines with cycle precision. (damn, I'm old)
It's not like it's hard to find a harmless 3 cycle instruction.
For example Store A/X/Y to any unused/scratch Zero Page location.
Or condition branch to the next instruction using a condition known to be true .. BCC/BCS is often the most convenient. Or just JMP to the next instruction, though it's 3 bytes of code instead of 2.
INC/DEC of a scratch Zero Page location is useful for a 5 cycle delay in 2 bytes of code if you don't mind changing NZ flags, but the store/branch/jump alternatives don't even do that.