In British English it is totally reasonable and in fact relatively elegant, in American it might take a couple of seconds to figure out what was being said and it would seem ungainly for this reason.

On the British elegance: in British English one could of course use "thought to be extinct" and in these kinds of structures it has become common as a stylistic touch to drop the helping infinitive, "to be", considering it implied.

I don't usually use this structure, but my mother did, and she was an awesome writer. She was British, so it may be more common, over there.

Yes but they could've used the word "considered" instead.

"Possum considered extinct for 6,000 years"

It's a bad sentence. They mean "thought to have gone extinct 6000 years ago". But some allowance for headline constraints should be given.

Yes, with the caveat that it's the distinct dialect of Headlinese

yes I recognize that as legitimate grammar, in a literary way.. It can be spoken that way, too..