grammar - Are contractions "open"?

My apologies if this has been asked and answered before.

I know that word classes can either be open or closed; for example, nouns are an open word class and allow for new nouns to be created to communicate new meanings.

I also know that contractions are not a part of speech in themselves, but I recently had a debate with a friend over whether it was grammatically correct to create new contractions the same way other words are. I was arguing that it is acceptable, and I wanted to use the point that contractions are "open" as nouns or verbs would be, but that isn't quite right. Is it grammatically correct to create new contractions? If so, what grammatical rule or property would allow it? The word in question, if it helps, was "how're", as in "How're you doing?"

Thanks for your time.