If you're brave or stupid enough to do this, you can keep Calibre Sync working in the meantime by editing the metadata.db file before syncing to temporarily restore the deleted fields. This will only work if you access the library folder directly e.g. with WebDAV, i.e. not through Calibre web etc.
1. Close the Calibre app and make a backup copy of (each of) the metadata.db files you have - one per library folder. Preferably have a backup each folder in its/their entirety.
For each:
2. Open each of the metadata.db in a suitable database manager e.g SQLite Studio, DBeaver, etc. FWIW, I used DBeaver.
3. Open the 'books' table and add the fields: 'flags' (INTEGER, default value=1), 'isbn' (TEXT), and 'lccn' (TEXT). Note the latter is "LCCN" in lower case not "ICCN".
4. Persist your changes to metadata.db, then sync with Calibre Sync.
5. Delete the altered metadata.db file(s) and restore the backup copy(-ies) BEFORE accessing it/them from the Calibre application.
Cross your fingers and beseech whatever god or gods that you favor that you haven't screwed up you library(-ies) completely. On second thoughts, do this at the start.
