In my opinion I think it’s possible to infer by what has been said[1], and the lack of a 5.1 “Thinking mini” version, that it has been folded into 5.1 Instant with it now deciding when and how much to “think”. I also suspect 5.1 Thinking will be expected to dynamically adapt to fill in the role somewhat given the changes there.
[1] “GPT‑5.1 Instant can use adaptive reasoning to decide when to *think before responding*”
[1] “GPT‑5.1 Instant can use adaptive reasoning to decide when to *think before responding*”