Last I saw, engineers working at OpenAI denied this on HN.
I saw that someone set up a tracker that aims to record the performance of the models, and so far it has not shown any statistically significant deviation in performance for Codex, and not yet enough data for Claude: https://marginlab.ai/trackers/codex/
The firm [Anthropic] would deliberately degrade the model’s performance in ways that were invisible to the user.
For such thing to be useful, it's enough that they works substantially more times that not having those instructions in.