In forum module, Drupal generates the feed url automatically, that is excellent. But the issue is, the feed url generated doesn't contain the facing slash (at the start of the url), because of this, the search crawler(bot) assumes the url as relative and leads to page not found.
Bug1: The default forum (General discussion forum) page url is '/forum/1' where '1' is taxonomy term id (which varies if you have some terms already before installing the forum module) and the feed url for that is 'taxonomy/term/1/feed' without the guiding slash at the front which leads to the confusion.
Bug2: When we navigate to the '/forum' page, the feed url still gets created and now it assigns '0' as term id ('taxonomy/term/0/feed') which is bug.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Forum is approved for removal. See #1898812: [policy] Deprecate forum module for removal in Drupal 11 →
This is now Postponed. The status is set according to two policies. The Remove a core extension and move it to a contributed project → and the Extensions approved for removal → policies.
It will be moved to the contributed extension → once the Drupal 11 branch is open.