What does the word "open" have to do with extensibility? Absolutely nothing. You had to phrase this as "open for modification" to make your point, why isn't "open" alone sufficient? My local museum is open from 10 AM to 8 PM daily, at least when COVID-19 is behind us. Does that mean I can bring my own exhibits, or remove the existing ones? No.
"Open" alone is fine. "Open source" has a specific meaning. "Free and open" is deliberately cribbing off of the well-known phrase "free and open source". How about this: why is "open and free" less suitable?