A client hands over a backlink report from 2019. Six hundred referring URLs, all recorded as HTTP. The site migrated to HTTPS two years ago, and now nobody can tell which of those links still exist.
Normalise before comparing
Convert the old list to HTTPS and compare it against a current backlink export. Matching rows are links that survived. Rows only in the old list are candidates for lost links — and now it is a real list rather than six hundred unknowns.
Do not skip the verification
Some of those links are genuinely gone; others merely moved. Check a sample of the apparent losses directly before reporting a number, because a redirect chain can make a live link look dead in an export.
What to do with real losses
Losing a link from a site that still exists is usually worth an email. Losing one from a site that has since closed is not recoverable, and separating those two piles saves outreach effort that would otherwise go nowhere.
Try it: Convert HTTP to HTTPS on SeoWolf's Notepad.