I want to highly recommend two tools for helping you with this very sad situation, which you must use before it happens.
The first is this wonderful script that will automatically add key information (that you choose) like author, title, summary, etc to your bookmark notes so you at least know what got deleted.
The second is this incredible bulk downloader for AO3. Input any AO3 page (such as your bookmarks page) and tell it what format you want to download in and it will just.... DOWNLOAD IT ALL!! I run it every few months so that I have all my bookmarks downloaded in my preferred EPUB format. That way I know I always have my favorites downloaded in case they ever get deleted.
With the combination of these two, I can look in my bookmarks, see what was deleted, and easily find it in my downloads. It's such a relief to have my own fic library is all my favs!
Let me add my life saver, an absolute wonderful add-on, ao3 saver:
https://addons.mozilla.org/en-US/android/addon/ao3-saver/
It automatically saves evey fic you visit so you have a backup if the fic is ever erased. No more mysterious bookmarks lost forever.
Also if you go to the page of an erased fic, it not only gives you the option to check on ao3 saver in case you have it saved, but also in archive.org in case someone ever did a backup there.
