* Rarely used (only used ~15 times in total, not used at all since 2015-2016).
* Merging topics didn't properly bump the new topic.
* Merging topics didn't log a modaction when the old topic was deleted.
* Merging topics broke the old topic. Moving all the posts from one topic
to another leaves the old topic with zero posts. This normally can't
happen and it causes exceptions when you try to view the empty topic.
* It was technically possible to merge a topic with itself. This would
break the response_count.
* It was technically possible for a mod to merge a topic into an
admin-only topic.
Few people used forum subscriptions (only around 100), and even fewer
people were subscribed to active threads. Most subscriptions were for
old threads that will never be bumped again. The implementation also had
a few problems:
* Unsubscribe links in emails didn't work (they unset the user's
receive_email_notifications flag, but forum subscriptions didn't
respect this flag).
* Some users had invalid email addresses, which caused notifications to
bounce. There was no mechanism for preventing bounces.
* The implementation wasn't scalable. It involved a daily linear scan
over _all_ forum subscriptions looking for any topics that had been updated.
* Fix inconsistencies in how wiki pages were linked.
* Link directly to the wiki instead of to a title search that is expected
to redirect to the wiki.
* Remove the single alias and implication request forms. From now
on, bulk update requests are the only way to request aliases or
implications.
* Remove the forum topic ID field from the bulk update request form.
Instead, to attach a BUR to an existing topic you go to the topic then
you click "Request alias/implication" at the top of the page.
* Update the bulk update request form to give better examples for the
script format and to explain the difference between aliases and
implications.
