Heading tags in the Content App (currently using TinyMCE) should be removeable
If you use headings (H1, H2, etc.) in a Content App, and probably anywhere else that uses TinyMCE, then current configuration does not allow you to remove those headings. You can change from one heading to another heading, but that is all, unless you use the source code.
Either remove the Headings tool from the toolbar, so users can only use the Template Styles which can be cleared by "Tx", or change it to Formats and include the paragraph tag.
TinyMCE documentation: https://www.tinymce.com/docs/configure/content-formatting/