So the issue here is that these slopes have Non-Ramp Sector (effect 14) set on them. Previously this only disabled stepping down. (You can see Tails tripping down a slope in the GIF below.) One of the changes I made in !1192 (merged) was letting that sector effect also disable stepping up. I thought this might be somewhat more consistent too since the old step up code didn't actually run when effect 14 is set. But I guess it ended up working by some other means.
There are two solutions we could take here. I could make it so effect 14 doesn't disable step up, only step down. Or leave it as is, since it's a given to me that the effect will also be removed from the map itself.
To argue in favor of solution 2, I think the ability to disable both step up and down on small ledges for instance is more useful than disabling step down alone. It also only really breaks slopes, but I don't think you'd use this effect on slopes intentionally anyway since it breaks walking down them.
It says on the wiki that Sector type 14 (Non-Ramp sector) is intended to only disable stepdown. (See the end of the first paragraph here: https://wiki.srb2.org/wiki/Non-Ramp_Sector)
This breaks maps that use Non-Ramp Sector stairs, where they don't want people snapping down stairs, but they want to make them act like stairs when walking up them.