Then it’s a valuable record and should definitely be posted as an observation. But there’s no need to use it as a default taxon photo.
I think the vertebrates caveat was because it’s common practice to put pinned specimens of insects as the taxon photo if no live images are available. I’ve not seen this done with vertebrates so much, presumably because there are few if any vertebrates that require being killed and dissected to identify, while there are many insects and spiders that require this to reach a definitive ID. Again, the question is what image will be most likely to help someone else with an ID. A roadkill squirrel with its head missing is probably not going to serve as a helpful comparison image to anyone. A pinned moth with its abdomen missing because it was dissected still shows all the useful external features someone might want to see.
I totally agree with posting observations of dead animals including gruesome ones. As you said, it can be valuable info. I identify dead rabbits (or parts thereof) often. Sometimes it is a bit too much for me (rotting or intestines lying about) and I quickly check reviewed and move on.
Some people want to avoid seeing these observations and will exclude animals labelled as “dead” from the pool of obs they are identifying. We’re talking about photos used to represent the taxon which people can’t filter.