I actually just posted a related topic yesterday:
https://forum.inaturalist.org/t/extant-vs-extinct-observations/9859?u=peywey
So long as it can be proven that your observation contains an organism, extant or extinct, it has a place on iNat. Just bear in mind that extinct organisms will be flagged as lacking recent evidence of an organism. It’s also possible many people won’t bother to ID it. And many IDs for extinct organisms will only be as accurate as genus level. Though the observation won’t ever become research grade, it is al waa says nice to have days there that may pertain to the natural history of living organisms.