[OJFord]

I'm not sure it is really though is it?

It's legitimate in the same way that '451: we want to load up our site with so much javascript your computer catches fire and burns your house down, but that's illegal in your jurisdiction so this site is Unavailable for Legal Reasons' is.

To me 451 is like 404 (nothing here) and 410 (was something here but now there isn't) - it's a further progression/specificity to 'was/sort of is something here but it's not presently available for legal reasons'.

Common obvious one is DMCA takedown notices, but it might also be an injunction, or even accidentally published while still under embargo - or deliberately to prime SEO (not advice) - or NDA, etc.