I put together this mind map to represent approximate relationships I see between tags. Many tags do not fit well into a tree structure, so I've taken liberties in a few areas.
Hopefully this is useful for writing tag wikis and excerpts, and for pruning the less useful tags on our site.
I think this is great as an overview of our tags. I see several tags I wasn't aware of. There are a few tags that I personally would have placed differently, but with so many cross-relations between concepts that is inevitable. For most tags I think it's fine how you modeled it. There are two tags however I feel more strongly about and would like to see differently (primarily because they are topics I have much interest in myself):
ecological-footprint: a common misconception is that ecological footprint is a generic term used to describe impacts on the environment, so a synonym of environmental impact. However just like carbon footprint, the ecological footprint is actually a methodology (see also https://www.footprintnetwork.org/our-work/ecological-footprint/). So in your schema I would switch environmental-impact with ecological-footprint
green-it: for me green IT is the overarching topic of making IT more sustainable, or using IT to improve sustainability in other areas. So I see computing (doing calculations on a computer) as a sub-topic of green IT
The next question is of course, how can we use this? You mentioned pruning tags. We could focus primarily on root nodes for tag wiki improvements and look at leaves as candidates for removal. On the other hand, if a leave node has say 20+ questions, why would we want to remove it? Perhaps the scheme is most useful to identify synonyms? In any case I personally think that (most) changes to our tags should be community decisions. In my opinion this is the hard part because we generally have little participation in meta (see also your related question "How long to wait before enacting decisions from Meta?")
