Sometimes it's the case that the platform will tell you that one of your backlink is 'lost'. This means that a backlink you once had no longer exists.

This can happen for many reasons; the webmaster may have removed your backlink, the page your backlink was on may now return a 404 response, and other reasons.
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We use a provider called Majestic for our backlink data and they can sometimes return false positives. There’s no way for us to ‘double check’ these for you, and because this feature can still be so valuable we still include it for our users. 

One common reason is that the page we saw you had a backlink on now 301 redirects to a new URL. In this case you'll still see the backlink, but our system will not as the page 301 redirects.

It's still very important that you check these lost links, as there may be a very valuable link you need to be aware of. But be aware that some links may still be live. And if they are that’s a good thing :)