This is a bit of a weird one but I'll explain as best I can:
After a redesign, content was migrated and mapped to a new URL structure (same domain). However, there are still a lot of URLs 404ing that don't necessarily have a logical place to 301 to on the new site - for example images, attachments, and pages concerning content that does not exist on the new site.
I don't really want to just redirect all to / as I thought it might 'dilute' that page semantically somewhat and impact SEO. I also don't want to just leave the pages 404ing as there is quite a large number of them. This is also obviously not ideal for users.
I was thinking that a potential solution could be to 301 the URLs to a page that would content on it saying "Sorry, that content is no longer available" (like a 404 page would) and give some navigation options, but the page would actually return a 200 status code, as it'd be a normal page.
Is this naughty? I think it handles the UX issue in the best way as you're not just dropping people off somewhere random without explanation, but I'm not sure if it would be considered to be bad practice. If anyone else has had the same issue I'd be grateful to hear how you handled it!