I played this on steam near launch and loved it, but at some point they put out a big overhaul that dumbed down a lot of core features, making the game markedly more approachable at the sacrifice of self accomplishment for the player and other things. I'm curious if this version got the same update or if it's possible to roll back to an older version on GOG (I think I saw that option for a few of my games here)?

A key indicator of the overhaul change (in case you're unaware) would be character upgrades that are tied in with masteries (achieving a higher mastery would normally reward the player with a passive bonus of some kind; typically damage to monster type, improved healing, etc.)

I really miss the original vision Arrow Head had for this game. I'd love for a chance to play it that way again.

yes it's slayer edition same as the game on steam and no you cannot roll back to old version of the the game.