Decision rule: missing, gone, or moved

Decision rule: missing, gone, or moved
Retiring addresses gets sloppy because it feels harmless. It is not. You are deciding what happens to every link and every crawl of that address from now on.
The rule I use:
— Direct equivalent exists? Permanent redirect to it. One hop.
— No equivalent, but a genuinely relevant parent page exists and a visitor would be served by it? Redirect there, sparingly. If you cannot say the destination answers the same need, do not.
— Content is gone and nothing replaces it? Serve a not-found. It is a normal status code, not a personal failure.
— Gone permanently and you want it dropped faster? Serve gone. Same outcome, stated more firmly.
— Never redirect a mass of dead addresses to the homepage. It gets treated as a soft not-found anyway, and you lose the ability to see what died.
Then confirm your error page returns the status it displays. A page saying "not found" while returning 200 is the worst of both.
Dead is fine. Lying about dead is not.