I have been looking for a solution for sometime now. Have not found any one yet... All we all know every page should have only one H1 tag, however it is not known for most of the non tech editor. I want to restrict the heading option. So there will be no confusion. Better yet, if we can change the label to L, M, S for H2 H3 H4. It will make more sense. one article should not have more than 2 sizes of heading in fact.
<nav>have internal heading hierarchies as of HTML 5, and some blocks use those tags ( I hope your theme does too ). What you're asking is going to be very difficult to do, if it's possible at all, without building new replacement blocks. If you did do this, it would be immediately obvious in the outliner and document map at the top too. You also have the problem of confusion as text size is already an option in the block inspector sidebar