Are all combinations of language codes and regions in the language-subtag-registry valid?

Question

RFC 5646 (https://www.rfc-editor.org/rfc/rfc5646.html) and IANA language subtag registry (https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry) describe and list the language and region codes that make up the tags for identifying languages, respectfully.

My question is:

• is every combination of language subtag and region subtag valid?

For example, these are obvious variants of English:

• en-GB is "English as spoken in Great Britain"
• en-CA is "English as spoken in Canada"
• en-US is "English as spoken in United States of America"

But is this valid, or is this combination not allowed because it is not recognized as a valid variant of English?

• en-CN is "English as spoken in China"

Thanks!