I think I know exactly why the tail is most likely to not happen though.
Is not because some obscure lore bit says “no tails”, it comes down to the character model
You can think that with vulpera that shouldn’t be an issue but, considering vulpera came much later, that doesn’t mean anything to worgen since they’re an older model.
If you look at the base races models before and after the upgrade, you’ll notice that the polygon count was increased but, all in all, what got the most upgrade was the textures but, other than that, the model itself went mainly untouched.
To add tails to worgen they’d need rework the model entirely. Simply sticking a tail could lead to a lot of glitching (and C’thulu knows we have plenty of issues with that as it is with almost every race) and then the modification of the animations to prevent model splitting and add movement to the tail.
It wouldn’t be that easy (even though for a big company it should be…) but, if they won’t fix the proportions of male night elves, I simply don’t expect them to add tails to worgen.
He kind of does but it’s… questionable at best.
The only confirmation it’s Christie Golden saying she shouldn’t have given Genn a tail in Before the Storm. That’s literally it.
There’s no explanation as to why this is the case.