The message may be referring to old note revisions — old note revisions described as "items" in the sync status message.
Note that whenever an item (note, folder, tag, etc.) is deleted locally, Joplin should add a message to its logs tagged with "DeleteAction". For me, when I filter Joplin's log file for "DeleteAction" matches, I get:
11-26T15:34:27: DeleteAction: Revision.deleteOldRevisions: ; Item IDs: ["5fb7e20679004a6982e35330769d8f2c"]
11-26T15:34:27: DeleteAction: Revision.deleteOldRevisions: ; Item IDs: ["2afea47b0948481c8e8ffa53b92c2d3e"]
11-26T15:34:27: DeleteAction: Revision.deleteOldRevisions: ; Item IDs: ["3cbd2c64dd2940658353e827ffee6698","eabbbabab74145289fdce3d8f8ad5ee8","964aa4bdb1ae411bb6f427a2ec412802","1668a14f06e844c789ca2928e3f07c2f","17cd541b19374d39b2cae2539aadac4c","f08cae87a95e4e3994991c028bbc7af2","1e51f13e4a8c4fdbac96d14cb55f17d7","266cdbdff280469bb8f1fa343002421b","791fc788d0664b02a8d0c1c80632ca00","06ec63e1bba94840afca9409e55b8490","ebbe32175fdc40e2a34fcdc11d9ea0b2","f54ae6b56a2d445fbe20a8e16a93b89c","0b4b1348ef0744e59d7d17ab6a885bf4","1539db48250b4e4eafdb6b343fe2c843","3c9f99fcda6c46ae83984d3ec605d125","7220fb77906a41e99c18f170b01f7d8e","095a705bbe1a4915b0071742db98ea33","8eadcfc21d6643ca88216e778f1ce15c","9ff74904a23940e0ae1a51b3bca55a0b","2791cdede57c42d386e0063a469c90a7","b8f2021ff13c43a7976a082af2a20f68"]
11-26T11:50:13: DeleteAction: sync: deleteLocal: ; Item IDs: ["9b2e7d9c3e8c4c329a4c8e58ecce8f07"]
11-26T11:50:13: DeleteAction: sync: deleteLocal: ; Item IDs: ["80424eeb57f0491faa0ae5ab145dc16a"]
[... more lines ...]
Above, lines that include Revision.deleteOldRevisions correspond to old revisions of items that were automatically deleted based on the "settings" > "note history" > "Keep note history for" setting. Lines that include sync: deleteLocal: correspond to items that were deleted by a sync (e.g. old revisions that were deleted on some other device).