Does anyone know of new car stereo decks that have the old loudness button on them?

Pioneer I believe. The Pioneer decks from 10 years ago had a physical "loud" button where you can switch between different levels of it - pretty much boosts the upper and lower most frequencies. New Pioneer decks still have this feature built in but it's probably going to be in a scrollable menu rather than a dedicated button.

The old "loudness" button was simply a boost to the bass.

It went out sometime in the 80s.

Modern car stereos have built in graphic equalisers, fiddle with that until you get the sound you want. Then the EQ button will have the same effect as the old "loudness" button did.

It's rare to see a loudness button, but most new head units have a loudness setting in the audio control menu.