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

I imagine manufacturers stopped including it to cut costs, figuring correctly that most wouldn't complain. Like erik, I have it, use it when appropriate.

Just for kicks...  Check out Luxman integrated and control amplifiers.  They have well designed Loudness circuits.  

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