Why this not works?

imul r8,rdx,dword 0

It was decided in some point in time that imm operands cannot be forced to a size larger or shorter than the other(s) operands.

I'll not move this to Main yet though, because there is something "unfair" here. The following is allowed and makes a difference:

Perhaps this "unfairness" was discussed already but I'm pointing it out just in case it wasn't really intended.

You can always put some bogus value to force immediate's size and then overwrite that immediate, as in this example:

Yes, it was discussed before. You cannot multiply qword by dword, so this combination is not allowed just for the same reason as "imul eax,edx,byte 0" is disallowed. The "dword" case is allowed, because the size is right in this case, and is just a redudant operator. For 64-bit prefixing with "qword" works where you want to have full-size immediate, analoguosly, however the "imul" instruction doesn't have a 64-bit variant with full-size immediate.