The following special pages appear to be missing help links. This is //not// an exhaustive list. The suggested target for each is also given. I'll leave this for a few days for any feedback about the targets before uploading patches.
[x] Extension:AbuseFilter (2):
Special:AbuseLog, Special:AbuseFilter -> [[ https://www.mediawiki.org/wiki/Extension:AbuseFilter | Extension:AbuseFilter (on mediawiki) ]]
No dedicated "help" page currently exists; once such a page is created the link can be changed to point there instead
[x] Extension:Disambiguator (2):
Special:DisambiguationPages, Special:DisambiguationPageLinks -> [[ https://www.mediawiki.org/wiki/Extension:Disambiguator | Extension:Disambiguator (on mediawiki) ]]
No dedicated "help" page currently exists; once such a page is created the link can be changed to point there instead
[ ] Extension:FlaggedRevs (6):
Special:AdvancedReviewLog, Special:PendingChanges, Special:ValidationStatistics, Special:StablePages, Special:ProblemChanges -> [[ https://www.mediawiki.org/wiki/Help:Extension:FlaggedRevs | Help:Extension:FlaggedRevs (on mediawiki) ]]
[x] Extension:Graph (1):
Special:GraphSandbox -> [[ https://www.mediawiki.org/wiki/Extension:Graph/Guide | Extension:Graph/Guide (on mediawiki) ]]
[x] Extension:MassMessage (1):
Special:CreateMassMessageList -> [[ https://www.mediawiki.org/wiki/Help:Extension:MassMessage | Help:Extension:MassMessage (on mediawiki) ]]
[x] Extension:ORES (1):
Special:ORESModels -> [[ https://www.mediawiki.org/wiki/ORES | ORES (on mediawiki) ]]
[x] Extension:TemplateSandbox (1):
Special:TemplateSandbox -> [[ https://www.mediawiki.org/wiki/Help:Extension:TemplateSandbox | Help:Extension:TemplateSandbox (on mediawiki) ]]
[x] Extension:TimedMediaHandler (1):
Special:OrphanedTimedText -> [[ https://commons.wikimedia.org/wiki/Commons:Timed_Text ]]
No dedicated "help" page currently exists on mediawiki; rather, [[ https://www.mediawiki.org/wiki/Extension:TimedMediaHandler | the extension page there ]] links to commons for the feature overview, which is likely to be of more help than the technical documentation on mediawiki
[x] Extension:WikiHiero (1):
Special:Hieroglyphs -> [[ https://www.mediawiki.org/wiki/Extension:WikiHiero/Syntax | Extension:WikiHiero/Syntax (on mediawiki) ]]