For document details and annotations, the short answer is no. The file attachments the short answer is yes.

The more complete answer is that we have several copies of every document, the 'main' copy which is used for syncing and several additional copies that get created by the systems which generate recommendations, the catalog pages at www.mendeley.com/research-papers and other uses. The 'main' copy gets deleted as soon as you remove it from your library and click Sync. The other copies will eventually get deleted.

We also have backups of our server data on hard drives and tapes. These get replaced gradually.