Okay, I'll take a stab at this one.
The many I have visited, played on have been very popular even with level 6 players.
Level 6 players won't care about level progression
Whats the big deal if a player wants to level up when learning the game?
Learn the game on a Poundamonium server... right...
The fleshpound is one of the highest health specimens in the game, and is typically used by people to level up their demolition perk using maps that are well built to bunch fleshpounds together on pipebombs.
The reason they are so popular is the same reason maps like Doom 2 were popular. People can cheat their way up to higher levels by exploiting. Its like getting item drops in TF2 by idling, your getting an unfair advantage by using exploits over players who play the game legit. No one with any sense argued with Vlave when they took the stance that idlers were cheaters.
TWI and KEPT make sure maps aren't made to give players this unfair advantage,
but we found alot of maps were being dewhitelisted because using these mutators made them alot easier to exploit. This isn't fair on genuine mappers who worked hard to make their map exploit free and would have recieved a pass otherwise...
If I were to be blunt (and this is unlike me as I usually don't go on the offensive) but I wouldn't be surprised if the entire reason you are complaining is because traffic in your server is going down because people can no longer use it to grind their levels unfairly.
So yes the poundamonium mutator is dewhitelisted, and no it won't ever be put back on the whitelist.