I was surprised when I wanted to tag my question with nikon-d850 as there isn't any such tag. But there is a d800 tag. Is it reasonable to expect a d850 tag, for a camera that came out in 2017? Or is the lack of a tag indicative of the need for one?
Tags are usually create as needed by editors with enough privilege (300 rep pts). I edited your post to create the nikon-d850 tag.
I'm sure there are a few existing questions that can benefit from the tag. There's nothing wrong with adding it, considering there are several existing model-number type tags.
However, most times the generic nikon tag is more appropriate, because the question will often apply to models such as the D810/D810A, maybe the D800/D800E, and maybe the D5, or even including other mid-range Nikons.
My recommendation is to not overthink it. If it's logical, and there's already a similar tag scheme, go ahead and create it. We can always create tag synonyms and clean up later if the tag(s) aren't useful.