The parent theme inside class-visual-editor.php lines 371 to 375 removes obsolete tags. I.e. H1, which I need to reinstated (at least for posts and custom posts classic editor). Is there any way to do it via child theme?
Please note that H1 is applied by theme on the main page/post title. It should never been used second time on any HTML page from accessibility point of view. If you need to style a text the same way as H1 tag, the theme provides predefined CSS classes you can use – the h1 class in this case (should be available via “Formats” dropdown button).
But if you really need to reintroduce H1 in TinyMCE, this code in your child theme‘s functions.php file should do the trick: