I was watching an interview this morning on Tuneup Guitars. The luthier mentioned he builds some instruments with a
Spanish heel. He said the nice thing about the Spanish heel is that if the instrument ever needed a neck reset you would only need to take off the back to adjust the neck. Is this building method only used on classical guitars?

Eh, it's not that easy. It's actually quite an ordeal if you ever need a neck reset as taking off the back is one thing, getting everything to line back up is quite the task (I have done this).
That said, I think the wonderful thing about the Spanish heel is that it reduces the likelihood of needing a neck reset as the neck is not just constantly being folded forward. The heel against the back helps keep the guitar from collapsing in. I suspect the main reason it's not standard is that Martin didn't do it. And, most things these days are still copying the Martin design.

FWIW, one of my finest steel strings has a Spanish heel. It's an extremely lightly built koa guitar. So light you can see the shape of the braces in the top wood. But, it's around 22 years old and shows no signs of deformation from the body folding inward.

Thanks for the reply. Maybe I need a classical guitar.