This was available in the skin's TimelessTemplate.php up to 1.31
$html .= Html::rawElement( 'form', [ 'action' => $this->get( 'wgScript' ), 'id' => 'searchform' ], Html::rawElement( 'div', [ 'id' => 'simpleSearch' ], Html::rawElement( 'div', [ 'id' => 'searchInput-container' ], $this->makeSearchInput( [ 'id' => 'searchInput' 'placeholder' => $this->getMsg( 'timeless-search-placeholder' )->text(), ] ) ) .
I tried to apply to 1.39 version of the skin (by also adding below line in en.json)
"timeless-search-placeholder": "Search {{NUMBEROFARTICLES}} pages",
But I got parsing errors. Can someone recommend the best way to bring this functionality back?