https://www.pcgamer.com/you-can-finally-customize-staircases-in-the-sims-4/
dated September 5, 2019.
The Legacy Edition was released on November 12, 2019.
I don't know whether you always kept your game up to date, or you took some time off playing and came back to the pleasant surprise of modular stairs, but they were in the game before the LE existed, so the LE has that feature. It does not contain any new content created after the LE was released, including platforms.
I dunno what's up with that, then. This game has become such a headache for one reason or another that - till last night - I hadn't bothered cranking it up since June. Months-long gaps like that started becoming more and more common after "Strangerville" dropped, but I coulda sworn I had to switch to the Legacy Edition long before November of last year.
Maybe I DID mistake when the modular stair update dropped as a result of one of those gaps between playing. At this point, I have no idea. I just know that I won't bother spending anymore money on it, even if I DO upgrade my software in the future. I'll just build like I always have since TS4 dropped, or I'll go back to TS2 for my best ideas since I have the necessary build tools in THAT iteration without having to beg and plead for them, or the things to run that version of the game so I CAN have them in TS4.
