My understanding, which is limited, is that anything that is from the 1929 Tintin will be public domain, but the later additions are still under copyright. So y

My understanding, which is limited, is that anything that is from the 1929 Tintin will be public domain, but the later additions are still under copyright. So you can use the character, but not reference later stories or characters introduced later. It’s much the same as how you can freely make fiction about Thor, but you can’t freely make fiction about Thor, the Marvel character.