People are denying it because it's simply not true and for sure has not happened consistently.

In my own work I have not once noticed a degradation where I could have said with any certainty that it's not within usual deviation or me having a bad day.

Conspicuously the model seems to be especially dumb and annoying when I did not get a lot of sleep the previous night.

When OpenAI and Anthropic engineers post here on HN that they do not quantize models and silently replace checkpoints in the API, I believe them.

I saw nothing that convinced me the people who are constantly making these complaints don't just mistake any suboptimal result for changes in the model.