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

Okay, does this solve the problems? From what I can tell this still increases slope stickiness, probably even more strongly than before, but without introducing issues in evilmike211.j2l or eztrig.j2l. (No other filenames have been mentioned as problematic, so those are the only ones I've looked at.) It probably also gets rid of the "slamming" effect mentioned, but that's just me guessing what that might refer to. That said, there's always the chance this introduces new issues in other areas related to floor detection, so please try it out thoroughly.

A handful more levels have been identified as suffering in 5.12's slope physics: 1UPtssc, ICNAII03F, lowaaex, and NitrousCTF. In my experience, the build in the above post fixes all of them.

After about an hour of testing, i tested these levels + distopia + xlmss01 and they all seem to work fine. (XLMSS01 also has bug in the live 5.12 version when using the sucker tube from the top left and running into a warp unintended).

However, I couldn't find the related sucker tube for nitrousctf. I first assumed the tubes to base, but they work fine in either version?

Then I went to the two sucker tubes at the bottom area and found two bugs related to "waittime" in sucker tubes in both plus versions. Not sure if you meant these from the start Violet, but they are not working in the new version for me.

1. Sometimes it shoots the opposite direction (Y = 40 instead of Y = -40)
2. Sometimes it shoots instantly, instead of waiting

Bug 1 is in both the 5.12 version and beta 5.13 version, but not in the vanilla version
Bug 2 is in which i assume to be all versions (vanilla, 5.12 and 5.13 beta)

Originally Posted by FawFul

However, I couldn't find the related sucker tube for nitrousctf. I first assumed the tubes to base, but they work fine in either version?

The tube to the right base pretty reliably fails without this fix.

1. Sometimes it shoots the opposite direction (Y = 40 instead of Y = -40)

This strains belief. You're saying that sometimes sucker tubes shoot you in the complete opposite direction and nobody has ever seen this?

2. Sometimes it shoots instantly, instead of waiting

As you say, this has always always been how sucker tubes work: wait time is not triggered if you visit the same sucker tube event twice in a row, you have to visit any other sucker tube event in-between.

Originally Posted by Violet CLM

The tube to the right base pretty reliably fails without this fix.

This strains belief. You're saying that sometimes sucker tubes shoot you in the complete opposite direction and nobody has ever seen this?

As you say, this has always always been how sucker tubes work: wait time is not triggered if you visit the same sucker tube event twice in a row, you have to visit any other sucker tube event in-between.

That makes sense how the sucker tubes don't reactivate waittime on the same one, never noticed that's how they worked!

Somehow they shoot the complete opposite direction yeah, i suggest testing the level yourself to verify. I didn't compare it to a prior plus version like 5.11 yet... Realistically though how many times do we encounter waittime sucker tubes at all or during one plus patch

There's a mutator in PN Tests server that restores old slope physics by typing !slope on.

I tracked down the WaitTime problem: rather, the problem was that you could press Down and buttstomp while waiting for a WaitTime tube to launch you. In the case of a sucker tube that launches you upwards, this could be mistaken for a tube that sometimes sent you in the opposite direction. It's fixed in 5.12b.

Everyone who's tried this branch reports that it fixes the sucker tube problem in all levels, so we'll put it in the next release (5.12c?). Thanks all!