WikimediaUI icon standard set in OOUI features a white question mark on black circle background labelled 'helpNotice', for use in places like frameless icon-only buttons, Revision-Slider as one example.
It also has a black on transparent '?' icon 'help' help-ltr.svg for use in toolbars, VisualEditor's toolbar or Notifications's dropdown as examples.
Other, non-standard help icons in use:
MediaWiki core
- MW core's mediawiki.helplink RL module for the "help indicator" on wiki pages uses a slightly different version of this icon in resources/src/mediawiki.helplink/images/ – replaced 2019
Active Extensions/Gadgets/web
- Echo's former icon is on commons as File:Echo question.svg and is used by the ImageAnnotator gadget. Example…
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/resources/images/help.png (c0c4771eee4a517536f5055bdcd4d433c08e19ce / #410) and help.svg (014b5632fcc6a4200d519cf7f86e94c09639af8f)
- UniversalLanguageSelector – https://gerrit.wikimedia.org/r/468499
- VE's icon is on Commons as File:VisualEditor_-_Icon_-_Help.svg and has some usage, particularly on frwiki.