Hi and welcome to the developer forum!
I don’t think there are any official API usage stats available outside of OpenAI, typically those metrics are classed as trade secrets unless it would be of value to a potential client or buyer.
I can tell you that the number of developers on the forum keeps growing and it seems busier now than at any point in it’s history, other than maybe a few short periods just after a new model launch, when clearly there will be extra activity for people investigating. So if you go by the number of developers, I’d say it’s increasing.
If you note the wording of the Similarweb article, it states that “growth” is down 50% across “all” markets, but ChatGPT was only down 5% (I don’t have the article to hand to check those numbers but I think they are in the right ballpark) that would mean that growth has slowed less than the rest of the market for ChatGPT during the western summer holiday season, which is always a slowdown for tech.