I was also affected by the broken stairs, but my circumstances were different.
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 didn't work anymore had walls on both sides.
Something about the new platforms has jumbled the routing for staircases, I think.
@Plallo Try replacing your staircases and (if necessary, some minor remodeling) to see if it helps. My staircases work again, but only if they have a wall on one side and no more than one bend in them.
It's true that this was NOT a problem before the patch.
