On that page I don't see any mention of o3-mini. Is o3-mini a legacy model now which is slated to be deprecated later on?

Nothing announced yet.

Our hypothesis is that o4-mini is a much better model, but we'll wait to hear feedback from developers. Evals only tell part of the story, and we wouldn't want to prematurely deprecate a model that developers continue to find value in. Model behavior is extremely high dimensional, and it's impossible to prevent regression on 100% use cases/prompts, especially if those prompts were originally tuned to the quirks of the older model. But if the majority of developers migrate happily, then it may make sense to deprecate at some future point.

We generally want to give developers as stable as an experience as possible, and not force them to swap models every few months whether they want to or not. Personally, I want developers to spend >99% of their time thinking about their business and <1% of their time thinking about what the OpenAI API is requiring of them.