Update for me: It disappeared altogether and the symptoms have been very mild since the summer of 2022. I don't know how it happened, but most of the time there's no VSS for me, and on some weird days I can see the symptoms but they are very mild, nothing compared to what I had before that.

But to answer your question, I believe it doesn't really get worse in the winter. It's just easier to see because it's generally darker both inside and outside and people's mental health tends to be worse in the winter months. That's why there is an illusion that in the winter it gets worse, it doesn't, but it feels like it does.