I posted this as a reply to another thread but am posting it here, too, in hopes that it may help some of you.
I was also affected by broken stairs.
In the house I'm currently playing, the only staircase that wouldn't work anymore was one that had 3 bends in it. I'd created a spiral(ish) stairwell that connected the second and third floors of the house. THAT stairwell no longer worked.
Unfortunately, my sim was on the third floor when the patch was installed, so I had to remodel that stairwell completely so she could leave the top level of the house, since suddenly becoming Rapunzel wasn't quite what I had in mind for her storyline.
I have four other indoor staircases on the lot - two that each have one bend in them and two that do not have any bends. All of those still worked (aside from having snow on the railings even though they were indoors (???), which I was able to fix by deleting and replacing the roof.)
I'm wondering if the game is confusing staircases with platforms now in terms of routing. Remember they said there would be height restrictions on which platforms would be accessible to adult sims...maybe adults think they're too tall to use the stairs now? Hard telling...
Anyway, replacing/remodeling the staircase that didn't work solved the routing issue in my game. I had to change a few things on the floor above it since I had to change the way the stairwell was shaped, but the stairs did work after that.
I'll add that all of my indoor staircases ARE attached to a wall on one side, but not both. The one that stopped working had walls on both sides.
Something about the new platforms has jumbled the routing for staircases, I think.
Try replacing your staircases and (if necessary, some minor remodeling) to see if it helps. My staircases are working again, but only if they're attached to a wall on one side and have no more than one bend in them.
It's true that this was NOT a problem before the patch.
EDIT: Adding that two of my staircases ARE in "Open to Below" areas. Those still work, but they're attached to a wall on one side and have railings on both sides (even though it causes a bit of graphic weirdness on the wall.) 🙂 There is also a 90 degree bend at the midpoint in both of those staircases, which may or may not be relevant.
EDIT 2.0: Another thing that might be causing it...perhaps the game thinks that staircases in open areas need to be connected to platforms? I seem to recall reading something about there needing to be stairs between platforms and house levels in order to make them routable. Maybe the game now reads open staircases as unroutable unless they're connected to platforms. Not sure about that, but I could see how that MIGHT be an issue if there's some kind of platform to stair connection requirement that is also being applied to staircases in open areas when it shouldn't apply to them. (#NotAllStaircases 😛)
