I got all of this from second hand so I hope other players can back me up here.
Apperently the new slopes function breaks certain functions in popular levels, for example the 'tube' below the red base in Distopia. Additionally it breaks a lot of test levels.

It could just be me, but I swear that 5.12 is a lot more aggressive with slamming the player to the ground even on flat surfaces.
Why has this not been made an opt-out feature through a /command or the menu bar? The /quirks command exists for a reason.

Originally Posted by SpazZTRoN64

Why has this not been made an opt-out feature through a /command or the menu bar?

Basically, it was publicly tested for eight months, the response was favorable, and nobody found any problems with it in any levels. (Except for with speed destruct scenery, and that was fixed prior to release.) The proposal for three months prior to release was that specific levels should be able to turn it off, which they can, and nobody objected to that proposal. This wasn't on our radar as something that might have issues--as far as we could tell, this was something that the community looked at for months and seemed to like.

The /quirks command exists for a reason.

It does, though so far everyone seems on board with the proposal to turn /quirks into a server-side test level thing,. Once that happens it should definitely turn the slopes change off, among various other changes... I'm not sure yet if that means, when it's a test level, ignore the value of jjPlayersStickToSlopes? Or set it to false upon the gamemode changing? Exact implementation details tbd, there's probably a right answer we just haven't thought it through yet.

slamming the player to the ground even on flat surfaces

So I'm not sure what this refers to, but the absolute best way to observe potential physics differences between versions is .j2m files: if you have one of those, that would be great to look at