Craft 3 redirect of existing and non-existing URLs
Based on your comments, I'd recommend creating a new entry type in your section.

Create a "Redirect" entry type which only contains a URL link (or an Entry field) so the page knows how to handle the redirect. For any entries that need to be converted to a redirect, simply swap the entry type to use this instead.

Then you can simply map that entry type to a redirect template:

{% redirect entry.newUrl %}


I've done this, and it works well.