Hello,
is there a mod for browsing topics from all forum boards in one "virtual" board?
(list of topics from all boards sorted by last post date)
Such mod would have a noticeable impact on your forum's performance and might be confusing for your users if there are similar topics on different boards.
As an alternative, try using the unread and unread replies features:
https://www.simplemachines.org/community/index.php?action=unread
https://www.simplemachines.org/community/index.php?action=unreadreplies
you can try this Recent Forum Topics on Boardindex (https://custom.simplemachines.org/mods/index.php?mod=3856) mod it will display the recent topics from all boards in a block above the boardindex list.