Easily rectified. Just allow polymorphs to talk no matter what.
What would be the fun of polymorph then?
The original idea behind polymorph was to neutralise the current weapon and armour of the victim (figuratively speaking stripping them naked, although the equipment was saved and was returned to them once the polymorph wore off). In return they'd get some animal attack (probably bite or gore) and a thicker hide (i.e. better ac). They'd show up on the who list and in the room as 'a boar, vaguely resembling to Once', and they would 'speak' in grunts or growls or croaks, depending on what they were polymorphed into.
I think the polymorph races were boar, bear, lion, wolf and toad, and which one you got from the spell was random. I'm not quite sure whether one race was stronger than the others, but that probably was part of the idea. When polymorphed you'd have one chance out of five to become a really strong entity for a while (the lion), and another chance out of five to become a weak toad. The other three races would have been pretty mediocre.
As far as I remember polymorph was taken out because Mordecai wanted to test how it affected players in a fight. So he polymorphed every player on line at the time, and transported them to the Arena to test it. A bit capricious, maybe, but pretty harmless, since you don't really die in the Arena.
Unfortunately he forgot to warn them about it first. And even more unfortunately one of the transported players was a Drama Queen, who felt 'offended' by it and started an endless rant about how degrading it had been, comparing it to rape, and other stupid exaggerations. The ranting went on for days and upset the entire mud, and I guess the imms tired of it in the end and just decided to take it out.
In retrospect it's amazing how many potentially cool features that were disabled, because of various drama queens who'd make an issue about something or other, and blow it all out of proportion.
As for the quest mobs, I don't see that as a big issue, unless some idiot would take it on themselves to go around polymorphing all key mobs, just to stop people from questing. And even then, a simple !polymorph flag in OLC wouöd resolve the problem.
The polymorph feature most likely was just commented out, meaning it still exists somewhere in our code.
I'm all for having it back, since the idea was originally mine. I wanted Circe on Aeaea to have it as a spell, true to the Odyssey story.