Turns out I had evidence in a book of mine...
The History of Armour 1100-1700, by Paul F. Walker. pgs. 89 and 90. Pg 90 has a photo of an original early 15th century sabaton in the author's own collection using an integrally hinged heel plate that secures with a leather strap and buckle. There's no confusion on that piece if it's a connected heel plate, it most certainly is.
Then on page 89 is an effigy of Sir Thomas Beauchamp. On the caption the author draws attention to the edge of the plates where you can see the division between the armor and the sole of the shoe. It extends toward the heel implying the existence of a heel plate. It's hard to see in the image, but it's definitely suggestive of a heel plate. These pieces combined with the manuscript illuminations to me prove that there were certainly heel plates in the Late 14th and Early 15th centuries. If they were the norm, I don't know. But it looks like the definitely existed.
I would scan the images and display them, but it's expressly forbidden in the copyright section of the book without written permission from the publishers, which I do not have.
This is an internet photo of the same effigy - (Dated 1369) - and you might be able to make out what appears to be a heel plate (I've since emailed Saint Mary's Church in Warwick to see if they'll take a photo for me, long shot, but the worst they can say is no):
