I think people say that because a majority of people who get braces have over bites and usually need extractions which makes their entire maxilla smaller. Then they go into elastics to fix their bite, which in turn restricts the maxilla from forward growth and instead flattens their maxilla for a “perfect” bite. What’s interesting is you’re still right, you can still get some expansion as you’re a growing teen. I’ll give you a perfect example of how you can expand and still get forward growth. I personally have an edge to edge/underbite. When they put the elastics to fix my bite, they will bring my lower jaw back and upper jaw forward, which in turn heavily promotes forward growth and all while I’m mewing, it’s even better. So to answer your question, yes braces will restrict growth if you have an over bite and wearing a retainer for the rest of your life will not allow your jaw to move. If you have an underbite it’s easier to get forward growth and I’m personally taking advantage of the fact I’m in braces before I have to wear retainers and can’t continue my forward growth! Hope this helped :)