[FIXED] Open staircase cause routing failures

@BluebellfloraHave two houses in gallery already in which my sims cannot get upstairs at all presumably because of "open to below" area with railing/fence as maysbat reported: The Summer Home and Coorinberg Cottage. I have plenty of other houses with open to below areas, which I bet don't work now either. The stairs were working perfectly before today's patch, and I was hoping patch would fix another issue, but when I tested, nope. Not only is the other issue uncorrected, but now Sims can't even go upstairs. (Replaced stairs, removed decor, deleted column, but it didn't help.)

Just tested, and with floor built, Sims can go upstairs and down fine. Remove flooring over foyer/living room, and Sims can walk all over upstairs balcony/landing even right to tile at head of stairs, but then can't use stairs. Haven't tried rebuilding entire rooms yet, but from what maysbat said, it will even happen with a new build. And, as practically all of my builds have open to below areas around stairs, this will make them unplayable.

Is there any way to uninstall patch so that I can actually play with my houses and Sims again?

@crinrictNo. Completely new one. In this one, upstairs hallway including place where stairs land is supposedly accessible to Sims, you can click and get option of "go here" with footprints, but the Sims cannot use the stairs. So, Sims stand at base of stairs waving their arms with a "can't go there" thought bubble. If I put floor where it should be open to below, Sims can get up and down stairs, but the house plan is ugly. Oh, and if they're upstairs, and the open to below areas are, well, open as they should be, they stand at head of stairs waving their arms with a "can't go there" thought bubble. Equally a problem upstairs as well.

The other issue was minor compared to this. At least I found workarounds for that, weird as it was that some parts of floor were inaccessible and had to change floor plan or roof to correct.

This one is affecting so many of my builds, I've warned people not to download my houses if they have the Nov. 10 patch because the stairs won't work.

Edited to add: I have used (multiple) workarounds to get my houses working with 11/10 patch, for the most part. So, most of them are fine now. But, man, was it a pain. The simple workaround is only half the battle because after that ceilings and rooms have to be adjusted to get the game to look and behave as though they're indoors again. And sometimes, even that isn't enough - I'm looking at you Chez Llama! To be fair, Chez Llama is indoors per the game except for some weird lighting issues.

Wow, this is really bitter and destroys a lot of my community lots. A simple stair like in the attached photo doesn't work any more

Product: The Sims 4
Platform:PC
Which language are you playing the game in? English
How often does the bug occur? Every time (100%)
What is your current game version number? Nov 2020
What expansions, game packs, and stuff packs do you have installed? Not relevant to issue
Steps: How can we find the bug ourselves? Create a room, then duplicate it and place the second one on top. Place a fence down the middle of the second floor and then delete one side of the floor. Then place a staircase which goes through the fence to create a lofted space. The staircase will place as normal, but now it will not function with the sim giving a movement error. Prior to the latest patch this was very commonly done and used to work fine but now sims are unable to use the stairs. In lots where the stairs worked before the patch, they won't work anymore.
What happens when the bug occurs? Sims wave arms around, thought bubble with a cartoon set of stairs and a cross through it and they won't use the stairs.
What do you expect to see? Prior to the patch sims walked up the stairs with no issue
Have you installed any customization with the game, e.g. Custom Content or Mods? Not now. I've removed them.
Did this issue appear after a specific patch or change you made to your system? Neutral/Not Sure 

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Following the latest patch, the trick to create a lofted room with stairs no longer works. The trick is where you use put a fence dividing two parts of a room and then delete the floor on one side, and put the stairs through the fence to create a loft space (like below). This previously worked fine and was widely used by players but now following the patch your sim won't use the stairs and will just wave their hand around with an error bubble. (see below)

It happens every single time and has now made every lot using this trick unusable

Am having same issue. I just went through everything I could think of (except bulldozing the lot) trying to get the stairs to work. On the same lot, the stairs leading to the basement function fine, but the stairs in the open foyer are non-functional. This is very unfortunate as 99% of my builds utilize open stairs. 😢