@me57701 Who? Me?
Mostly, I just had to control delete a section of the stair railing/fence at the head of the stairs, and, then, when the game put construction grade concrete over the "open to below" area, use a sledgehammer on it. Boom, sims can use stairs.
You can then replace the railing at head of stairs if you want and the game will put construction grade concrete in again. Sledgehammer go boom will work for that, too. And sims can still use stairs.
After that, some builds required more fixing up than others to get them to look okay/get the game to recognize them as inside, not outside, etc. (Chez Llama still has a bit "outside" lighting due to the way the Sims Team built it, but it is covered and inside according to my sims.)
One thing that is very common is that using the sledgehammer to delete the flooring on the "open to below" area also deletes the ceiling above unless there's already a full attic room with floor up there.
Drawing a fence up on the level where the ceiling has gone missing will create a "floor" there which will remain even after control-deleting the fence. Very important for the game to recognize that area as "covered".
Oddly enough, one or two of the builds I was most dreading ended up not needing much fixing, or indeed any, depending on the stair configuration (for some reason, Ophelia Villa was perfectly fine? Did I put in the stairs in a different way there? 'Tis a puzzlement).
However, some of them really went wonky (I'm looking at you, Affluista Mansion, and you, Hound's Head) and needed floor plan adjusting even on the lower levels to put walls back in a way the game recognized as enclosed rooms again. Think of the hokey-pokey!
Anyway, that was my completely non-systematic, panicked approach to fixing the builds in my gallery so that sims could use stairs again and the game would stop treating exterior walls (and some interior rooms) as "outside" with weird lighting, etc., and so that I didn't feel too guilty about people downloading my gallery stuff.
Mind you, I still think being able to delete the floor over the stairwell without "sledgehammer go boom" was a much more practical, elegant solution, and the platforms don't really make up for that loss, not to mention that it makes no sense why platforms messed that up.
If you're building from scratch, it's still a limitation, and that's not fun either.
