As documented at https://www.mediawiki.org/wiki/Requests_for_comment/Reimplement_info_action, the info action in MediaWiki was disabled by Tim in r4247 by putting it behind the global variable $wgAllowPageInfo and setting this global to false by default.
Now with years of hindsight, it's become clear that a more generic solution should have been used (e.g., $wgDisabledActions), however this bug is about killing (or deprecating) $wgAllowPageInfo and enabling action=info by default.
This requires marking the global as unused in DefaultSettings.php, removing the conditional code that relies on this global variable (fortunately it's not used in many places), and then disabling the expensive portions of the info action that caused the action to be flatly disabled so many years ago (namely disabling any properties that rely on unoptimized queries).
Relying on $wgMiserMode is one option. Another option is to introduce an array such as $wgDisabledInfoActionProperties or introduce another binary global such as $wgDisableExpensiveInfoActionProperties, though I'm very hesitant to add a binary global variable again.