The changes implemented in T13555: .mw-editsection links should not be part of the <h#> element (see "Heading HTML changes") are not enabled by default in MediaWiki 1.43, and have to be opted into by site administrators by setting $wgParserEnableLegacyHeadingDOM = false.
As documented in the release notes at the time, and on that documentaton page, in a future release we should make them enabled by default, and change the config setting to be an opt-out. This is tentatively planned for MediaWiki 1.44.