I think Hemingway mode is when backspace and delete are disabled. Ghostwriter (https://wereturtle.github.io/ghostwriter/) has Hemingway mode and another mode that they call "distraction free mode" which highlights just the current sentence.

FWIW I find it really distracting to have the current sentence highlighted because it means the file's appearance changes more than it otherwise would. Sublime Text has a "distraction free mode" (shift+F11) which goes to fullscreen, hides all menus, limits the current file width, and centers the file's contents.

Sublime Text has a Distraction Free Mode which goes to full screen, removes all GUI and centers the buffer contents on the screen.

It’ll use your buffer settings so if you’ve configured Markdown to only be 60 characters wide, it’ll look like the photo in the article.

If I understand correctly, the Limelight plugin for Vim does this. It's best used in conjunction with Goyo, which achieves the narrow columns as well.

Typora