It would be a little strange but not that strange for someone to have an idiosyncratic preference for GPT-5.5. It's still available as a Codex model (I think? It's selectable in Pi, not Codex itself), and it might perform differently enough from later GPT models that someone prefers it.

To your point, though, I don't know if I've noticed that big of an impact going from 5.5 to newer GPT models. If I had to switch back, I'm not sure how much I'd miss the newer ones. People talk about e.g. Opus 4.6 similarly, I've noticed.

The bigger change in OpenAI-land IMO is the models got cheaper, generally.