NHF – new heroes for Heroes V 3.1.

But why don't we see 89 in the game?

Added 2 minutes ago
As I understand it, it's simply not used, which means that to some extent, it can be considered empty, and we can take advantage of this by adding a new unit.

Not by adding, but only by replacing, which is not desirable at all.
If there isn't some kind of script ID, then it's not necessarily the case that the slot isn't occupied;)

On some standard maps, there are definitely black knights in the neutral creatures on the paths (like Key or something else, I don't remember exactly). Replacing them will lead to a change in the unit.
Yetis are not used in standard maps.