I've come across the tortadrones while reading the wiki, as well as slooms, and gold monsters such as trojans.
I don't know why these were taken out, but i'd really love a chance at them. Perhaps a new boss sloom? Or a tortadrone graveyard?
And are gold enemies a random chance like rares?
There are golden drone enemies, but again, they are rare monsters, and they spawn in certain fixed areas within missions, or anywhere within Clockworks Tunnels themed levels.
All of the other rare monsters such as the Golden Trojan/Gun Puppy, Tortodrone, and Sloombargo, have been removed from the game. I suspect they removed the golden enemies from the game because of them randomly spawning in areas that require you to kill every enemy, and not allowing players to move onwards. I'm not sure why the Sloombargo was removed, but the Tortodrone was removed since it dropped a four star shield, allowing players to farm this shield and easily earn 10,000 crowns each time they sell it to an NPC, or such.
Edit: Didn't realize that Slooms were no longer ingame until shortly after I posted.