The pull request (#654) was merged a little too quickly.
In particular, display_last_subject per-forum option needs consensus.
This reverts commit b1fb34a4d7e446768b9f66ba6dd359173cd2a5fb, reversing
changes made to 7d6b289671c51694f17dd5f9849abf2534b55e59.
PHPBB3-10650