I know from this answer that,

Modern x86 CPUs are binary compatible with 8086. You can literally run 8086 binaries on a modern PC, in real mode. (The species analogy is a stretch here, but works if you look at forward compat instead of backwards compat: old x86 chips can't run AVX / AVX2 / FMA / AVX512 code, so you could look at each ISA extension as a speciation event.)

But I also know that some instructions like the Binary-Coded Decimal (BCD) instructions by way of the BCD Opcodes are "not supported in 64-bit mode" (long mode).

What other instructions are not supported in 64-bit Long Mode?