I'm not at all comfortable with making this a blanket statement without doing more clarification. Where is the line between a prototype and an experimental change that you want to A/B test? We might have a prototype of a new button that we want to put out in front of a large number of users ("in production") and the only way to do that is to put it in the production codebase.

It is also perfectly legitimate for prototype code to evolve into production code. That won't always happen and you should be cautious about it, but we shouldn't deny it.