As ever ebook formats were initially only designed with novels in mind. Really most of the need for headings not starting on a new page is non-fiction, and even reflowable epub3 is still poor for scholarly works or textbooks, which is why those often use PDF.
I know about what you mention @RbnJrg, but fortunately it's only in notes / appendix of stuff I create that it would be needed and one option is to ignore it as long as the main content is OK
