The result of this is any site using the sole config variable $wgMFUseWikibaseDescription = true from before June 27th to show taglines who upgrades will now lose their tagline unless they turn on $wgMFDisplayWikibaseDescriptions immediately.
Find where $wgMFUseWikibaseDescription is still used and add the new config $wgMFDisplayWikibaseDescriptions = [ 'tagline' => true ].
Remove the old variables after the changes in T138788 roll out (21st of July).
- Wikis with old configuration variables and wikibase descriptions enabled keep the descriptions enabled as the config changes/deprecations take effect in rollout
- Old configuration variables are removed and configuration is up to date and lean
- The fallback deprecated config variables should remain in MobileFrontend to allow time for 3rd parties to catch up