Any insight into why mountain dwarves were removed?
Fluff-wise, the devs want to differentiate DC:SS from "standard Tolkien fantasy worlds", so Mountain Dwarves were cut in favor of Deep Dwarves as the Dwarf representative for Crawl.
Gameplay wise, it's a different reason (DD have the huge factor of being unable to regenerate normally which makes playing them MUCH different from playing MD, even though they are both Heavy Armor, big Axe-using kind of races.)
Here's a secret: MD actually suck (power-level wise).
Yes, I know there's that joke "chart" on how to win Crawl by picking MD and then wielding Axes and then pressing Tab at everything but the true "power behind the throne" was actually how broken Heavy Armour was in old versions due to damage reduction rounding issues. Once that's fixed and HA was "rebalanced", as it turns out Mountain Dwarves were terrible, especially compared to their closest competitors Minotaurs (and MD have one of the highest EXP requirements per level required for no good reason.)
The thing about melee characters in Crawl is that, as it is, there really isn't very many niches for races that are "Extremely Melee-centric, Heavy Armor, not great at Magic." One of the big reasons is being that "old-skool RPG devs" are extremely afraid of giving fighters/melee tactical abilities, because "choosing a weapon ability out of a menu" would turn them into "fighting-flavoured magic users" and the game would become WoW or whatever silly excuse they always come up with.
So out of all the humanoid races (and thus, very little gimmicks), you got MDs, Orcs and Minotaurs. Orcs have a god specifically made for them, and one of the few races that can play very specific kinds of "Heavy Armor Hybrids" (usually Fire or Earth Elementalists), so they're staying.
So you got MDs and Minotaurs left. Both of them are hopeless at magic (rivaled by Trolls, who are also a melee-centric race but "Monstrous" meaning relying on their own body/mutation strength rather than equipment) and good in all forms of weapons and armor, but Minotaur's horns gives them an extremely helpful early game edge that really helps buffer against the RNG nature of a Melee user's early game.
On a pure gameplay level, keeping MD around would basically mean "Here's a race that is almost 99% inferior to another, except in the rare chance that a super powerful artifact helmet is generated in the game, which MDs can wear and Minotaurs can't... if they manage to live that long."
Of course, at some level, the devs probably inherently knew they were going to get some shit that people were going to complain that "now there isn't a simple newbie beginner race any more for people learning the game" despite the whole 'actually MDs weren't that powerful' part, so they went ahead and buffed both of those races. Which actually sent Minotaurs over the roof into hilarity: they're one of the races with the most win rate right now, if not the highest. Well, until Gargoyles came along.
tl;dr: Just play a Minotaur and pretend you're a short beardy midget, there's no real gameplay difference.