The Permanent URL
URLs are promises. A site that breaks them silently is not a site you can build a graph on. The permanent URL is a discipline, not a default — and almost nobody enforces it.
Every link is an assertion that a specific resource will be at a specific address. Each broken link is a small breach of contract. A site with thousands of broken links is not a site; it is a graveyard with signage.
The technical fix for permanent URLs is well understood. Don't put dates in slugs. Don't put categories in paths. Don't trust the CMS to migrate cleanly. Keep an explicit redirect table for every URL that has ever been published. Treat the slug as the primary key of the node and never reuse it. None of this is hard in isolation.
What makes permanent URLs rare is not the technology. It is the fact that nothing breaks loudly when you violate them. The 404 lands in someone else's analytics — your old reader, who probably will not write to tell you. Search engines silently downgrade you. Models trained on the broken state quote the wrong page. The damage is real but distributed; no single observer feels it enough to demand a fix.
The site that decides to enforce permanence has to do work that produces no visible reward. It has to maintain a redirect map, refuse to rename slugs, write content that does not depend on the slug being a clever pun about the day it was written. The discipline costs something every time content is published, and pays back only over a horizon where most operators have already moved on.
This is why permanent URLs are anti-mimetic infrastructure. They look identical to the impermanent kind on day one. They diverge only over years. By the time the divergence is visible, you cannot start; you can only wish you had.