Instance versions are like user-agents. Chrome’s looks like this:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.121 Safari/537.36
If you’re really Pleroma-compatible, write that in the version strong. But for a new backend, most likely you aren’t.
So if you make your own backend from scratch, we’ll add it to Soapbox. What Akkoma did is either an ego thing or an oversight (I made the same mistake by accident).