Related Gerrit Patches:
|mediawiki/core : master||Remove $wgExperimentalHtmlIds and related code, deprecated in 1.30|
|mediawiki/core : master||Set default fragment mode to [ 'legacy', 'html5' ]|
It was deprecated by @MaxSem in rMWfd6e9ef2d481: Human-readable section ID support. Thanks to the deprecation policy we can't actually remove it until 1.31 or later.
Possibly someone should add a wfDeprecated() into the bit in Setup.php.