First off, deep always depends on the specific body of water. Bass always stay deeper in clear (rather than murky) water, often at depths of 20-45 feet. "Deep" would thus mean deeper than those depths.
Most largemouth in the south or mid-west clear water areas spend their days between 10-18 feet, and occasionally go to shallow cover or other food heavy areas to feed, but usually don't travel much deeper.
Bass actually go deepest in the winter, when the warmest water is found lower than shallow or mid areas.
The reason people tend to think the bass have gone "deep" is because they have gone "deeper" than spring. Spring means spawning, and that means the bass are in exceptional shallows, easy to find and easy to catch. As they move into summer, they move "deeper" but they still don't go "deep."
Lastly, even when the air temperatures reach the high 80s and low 90s, the water temperature in the region the bass typically hang out in is still in the low 70s. Anyone who has swam in a lake in the summertime knows this!