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

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. 

I have no loudness or tone controls. I’m sure others have explained better than I can why the shortest distance from A to B is an engineers goal. 

If you MUST have a tone variability option, tone controls provide more flexibility. Loudness was a cheap way to compensate for poor circuitry and parts in cheap components. 

I have one in my Dyna PASS. That was a long time ago. Dyna was good in their day. 

I used to have an ARC SP3a that had something called a "Contour" control. Basically, it was a tuneable "loudness" control. I don't recall seeing it on any subsequent models. If the tone controls were activated you could control the curve to add more bottom end for low listening levels. I never used it all that much. Being as this was mumblety years ago, I seldom listened at low levels anyways. But I could see it as a handy tool for apartment/condo dwellers or for background level music. 
Cheers...