After T371756: [1.45] Remove wgParserEnableLegacyHeadingDOM option to disable new heading HTML, MediaWiki does not output headings with the "mw-headline" class any more, only headings with the "mw-heading" class (see "Heading HTML changes"). Any code still using "mw-headline" needs to be updated or removed.
https://codesearch.wmcloud.org/deployed/?q=mw-headline&excludeFiles=%5C.%28html%7Ctxt%29%24
Previously: T323773: Remove unnecessary uses of <span class="mw-headline">