The router tag is mostly used on questions dealing with URL routing in web frameworks. I tried to estimate this, excluding many common web tags that I saw, and that reduced it from 3800 to 1400 questions remaining, of which many still deal with URL routing. So, mostly that.
The tag wiki however says "DO NOT USE THIS TAG FOR" URL routing etc.
The tag wiki then talks about routers as IP-layer network devices. That's networking. That is usually off-topic for Stack Overflow (configuration, operation, ...), unless you want to discuss it in a programming context. I'm taking bets on how many of these questions do that.
What to do?
- Re-dedicate to URL routing and wade through 3800 questions to find those that deal with routing that isn't URL routing?
- Double down, shoo away all the web dev people (and their questions) who will surely continue to use it for URL routing, since the tag wiki didn't work at all to dissuade them from using the tag?
- Give up, treat as multi-use/generic tag, change tag wiki to reflect reality?
- Wipe it from existence?
- None of the above?