Loudness - Why has the industry stopped producing amplifiers with this feature any longer?

Mitsubishi used a variable loudness knob on several of their receivers too, I always liked that feature over the loudness switch.  If you got a loudness button or dial, you end up using it.  For many more years than not, I don’t have loudness & I don’t miss it either.

I will not purchased a system without the loudness switch. I have a restored Marantz 2275 and the Vincent Audio SV 237 and both have loudness switch which are available when needed. My ears like music with the low-end forward and some of the older music are not mastered by today's standards and the loudness switch bring the low-end a step up closer. There is no right way or wrong way to listen to your music. It your equipment and your ears and preference. Think about this - every artist sculpt the music to their preference and the end results is what you hear. This is also true when a classic album is re-mastered by a someone other than the original engineer. There are good and bad mastered and re-mastered music that will always change the sound. The creator controls the final results.

My recollection is that the loudness button simply added an enhanced bass response.  Is my 20 to 30 year old recollection in error?

hayw, bass and treble.