How it worked on retail and how it should work are not the same thing.
And where I said something that contradicts it? Your post completely doesn't make sense from logical point of view.
Yes, in hypothetical scenario, the way something worked on retail didn't had to mean it was an intended way of it working. It's obvious and doesn't need to be mentioned, but once again, I haven't said something that would contradict this statement, thus your comment is an absolute nonsense and should be removed by mods to not kill gray cells of more people that could read it.
Regarding Backstab, I am pretty sure, the way it worked on retail and the way private servers emulate WoW in this aspect, is 100% compatible with, how it was meant and intended to work. The only way to be able to use Backstab is to get behind your target and wield dagger weapon in your main hand. Both conditions were obligatory on retail back in Vanilla times in order to use Backstab and same goes to modern days' private servers. Strafing over a mob with slightly delayed movement in comparison to player's movement, in order to get a small time window, when the player gets finally behind that mob and can use a Backstab, doesn't change this fact. You still need to get behind a mob and wield a dagger in your main hand to use Backstab. Everything corresponds to the tooltip of the ability.
What are your premises to think it didn't worked on retail as it should be? And what arguments/proofs you have to back those premises? Blizzard had 14 years to fix something you claim that don't work as intended, yet they haven't done it to this day. It makes me wonder about the state of your sanity.