But what about Dropbox Paper? Still beta, but headings are working nicely on all platforms. Sure, Dropbox Paper has a different editor, but at least it proves that it's not impossible to be implemented. I see no reason why it should be so complicated to insert a simple HTML tag into the HTML source code.
When Evernote grabs the content of a website, you can see all the heading tags in the source code. You can edit the eported .enex file with a text editor and add heading tags by hand! So why should Evernote's editor not be able to do the same?