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It would be really nice to have something like this for the x86-64 variant.

The same site hosts [1], but that's not nearly as nice as the 32-bit version. It's also a bit outdated.

Thanks. Looks like the original now has some clarifications, including more detail regarding the REX prefixes: https://wiki.osdev.org/X86-64_Instruction_Encoding

sandpile.org is your friend.