Honestly, I have yet to see any evidence of data leak from private sources. I think one of the better example is "simple-bench", which at least used to be a low-key benchmark that I would assume would have been saturated quickly if the labs were secretly scooping up data from API requests. Yet it's been years and it has yet to be saturated.

It's easy to catch a data leak if you have private data. You know what the model is supposed to not know, and you can just ask to see if it does. Yet I have not seen or heard of a single case of this being documented. As far as I can tell the labs do in fact respect the request to opt out of training.