I refer to the metallic ring surrounding a semiconductor die. (I have heard it called "seal ring")
This is usually connected to the substrate. The ring may also contain power lines in other metal layers.
Is there some best practices for how this ring is used and designed?
I see my colleagues only tie ESD cells to this ring. Could it also be used as a general power bus, or is this unwise?
If you have enough metals and if your ESD rails resistance is low enough (2kV or 4kV etc...) yes you can use it for general power routing.
It would be a pitty not used them and just leave them there...