Per Headbomb's comment, it's reasonable to be more confident adding links if we're also able to remove those which stop working, whether added by us or by others.
A potential start is around already_oa_param in propose_change(): it's worth verifying whether the target link is still up, before giving up searching OA links.
When we find a broken link/identifier, we can initially log it. Then we'll see what actions are appropriate.