In an older version of Kubuntu with KDE 4 it was at ~/.kde/share/apps/RecentDocumentsThere's nothing at that location in Kubuntu 15.10 with KDE 5, but recent documents are still tracked.
Where are recent documents stored in KDE Plasma 5?
kde5
I took a guess that the new directory might have the same name. I made sure my mlocate database was up to date, then searched for it.$ sudo updatedb$ locate RecentDocumentsI only had a few hits, and it revealed the location in Plasma 5.~/.local/share/RecentDocumentsI double checked, and the contents matched the recent documents in Application Launcher.