No it doesn't?

You can run stock, or any fork simply by providing the URL of the version you want to run.

Where exactly is the restriction?

https://github.com/commaai/openpilot/blob/master/docs/SAFETY... - they will ban your device from the training set if you run an unsafe fork.

That seems very reasonable, doesn't it? Otherwise the fork could pollute the training data through bad modifications, and tracing that would be a pain.

What? It's literally open source, you can ssh into the thing and change whatever you want. I am running a fork of a fork of the code right now. I change things all the time.