I don't know when Evernote introduced the large unsightly headers for merged notes. But I do know exactly when they started becoming more than just an aestetichal problem, more than 4 years ago:
The issue is, that the first header would be repeated if you merged notes with merged notes, see the example in the post above. A complete deal breaker for me.
I filed a bug report all those years ago, but have no hope of ever seeing it fixed.