I had to delete a library recentkt to reinstall it because some problems in it (some movies were being reindexed). So if I had had the possibility to export/import the list of my viewed films it would have been great.
Maybe also it could be a way to quickly check the list and reimport it in Plex…
If you want a ‘manual’ way, use Plex2CSV in the highest detail level.
It exports the column ‘viewed at’. If this is set at ‘N/A’ the item is unwatched.