TIL - ignore_routes_with_linkdown TIL, Today I Learned, is more of a "I just figured this out: here are my notes, you may find them useful too" rather than a full blog post
Usually, a high level network managers such as NetworkManager or Connman, removes routes for unaccessible ("linkdown" or "unplugged") link states.
If these links are not managed by a network manager but are managed in other ways or is somehow misconfigured, then you could end up with a default route that is still hanging around even if the link is down.