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

I sold Yamaha hi-fi in mid 70's.   They taught the concept based on the Fletcher-Munson curves and it worked well.  You set your maximum comfortable volume first, then turned it down with the loudness control.  (Variable, not just a switch).  I bet somebody to write an accurate digital version that could be an add-on feature for a miniDSP unit.

As others have noted, loudness controls (and tone controls) are out of fashion.

So, remember - loudness / tone controls = bad

All other manner of changing the audio quality of your system -- speaker wires, cables, etc.  = good.

Many AV receivers have the loudness control disguised as the Audyssey “Dynamic Volume Control” selection. While I never use it is there.... I generally just turn it up slightly to improve the perceived balance or live with the idea that there are other people I need to actually need to consider. 
If you use subs or full range speakers then put them on isolation/decoupling platforms and that will drastically reduce the transmission of the bass frequencies through the walls of your listening space allowing you to turn it up a bit to compensate. 
Or, (gasp!) use tone controls if you have them and upping the bass and treble by a couple of clicks on each end when listening at lower volumes.

Above all experiment... try things... be creative... don’t stop until you have something that works for you!

T

What's wrong with headphones? I won't use them because they sound so good I get carried away (and lose hearing in the process). Consider them if you've more self control.

For a few years , starting In the early 90s I had a Yamaha rx570 with a variable tone control, and Paradigm 9semk3 towers. The combo could sound thin and lifeless at low volumes; the variable loudness control worked wonders. In my second system  I have a vintage restored Tandberg 1055 with a well implemented loudness control. Very useful at low volumes, and seems to automatically compensate with volume . I use it without guilt.
   My main system has a tube hybrid Audible Illusions pre with 2 gain and 2 volume controls, and a high current SS amp, driving Maggies. For whatever reason this system plays very well at low volumes. Maybe its the tubes , maybe the high current, but it needs no "loudness" compensation. 
      High end audio has definitely gone "purist" for at least the last 30 years, and has eschewed tone and loudness controls. I understand the concept of a pure signal path, but I don't get my shorts in a bunch about using a loudness control if it makes things sound better for late night listening. If you like it, use it.
     Come to think of it, and this is just my speculation: a lot of audiophiles listen at quite moderate to low volumes for normal listening. Both myself and my closet audio buddy are case and point. Maybe that plays into the designs of some high end equipment builders, thus helping negate the need for controls.