The CPU also doesn't care if you use prefixes that aren't valid for a specific instruction, for example a REP on a non-repeatable instruction. The LOCK prefix is the only prefix that makes the sane choice to reject invalid combinations, rather than silently accept them.

Also, the (E)VEX prefix doesn't behave like the other prefixes: it must be placed last, and can therefore only appear once. All other prefixes can be repeated.

When AMD designed the 64 bit extension, they had run out of available single-byte opcodes to use as a prefix and decided to re-use those. The INC/DEC instructions are still available in 64 bit mode, but not in their single-byte encodings.

https://soc.me/interfaces/intels-original-64bit-extensions-f...

where there are links to a couple of patents filed by Intel in 2000, about a 64-bit extension of the x86 ISA, which had been implemented in Pentium 4, but which had been nonetheless disabled and hidden from the users, in order to not compete with Itanium.

The page explains the content of the patents.

As already mentioned by another poster, at least on Firefox you have to open a tab and then copy this link there, to avoid being identified as an "undesirable" :-)