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

The problem with a loudness function is that there is no way to set it up right. The idea is based on the Fletcher-Munson curve, which is variable depending on sound pressure. So the loudness has to be variable, the problem is knowing how to set it. With **all** loudness controls that are essentially a tap on a volume control with an emphasis network, there is a correct setting of the control with a particular speaker, and with a particular source. So if you have two sources that are at different output levels you'll need to have a different volume control setting to obtain the desired volume, but now the loudness emphasis is different. Obviously they can't both be right, and if you don't happen to be sitting the correct distance from the loudspeaker with which the amp was designed, it will never be right.

Now some amps had in addition to a volume control, a loudness control too, so you could variably set the emphasis; the problem here is knowing where to set the control. About all you know there is that if you have the volume set lower, you might want the loudness set higher?? But the reality is that you can't have any idea, not without doing a lot of measurements...

Obviously the result has been since their inception that loudness settings are a gimmick and nothing more. So high end stuff omits it since its entirely useless and only causes colorations. 

It is a feature I have seen recently in AV receivers and perhaps processors, but it wasn't labelled loudness. It is all done digitally of course. With many people having all digital systems, it does not make sense in the analog domain. At low listening levels, even digitized vinyl would be hard to argue against versus the alternative (not loudness compensation).

@atmasphere : Yamaha and Denon have both implemented variable loudness controls. (Not sure if Denon does anymore).

While I applaud your neurotic insistence on exact settings, the toggle switch Loudness button on my Luxman's sounds really great.

The reason it mostly went away was a big trend towards hair-shirt purism in preamps and integrated.  Tone controls, balance control, and loudness (button or variable) all were eliminated at the altar of the fewest possible stages and connections.  This is reflected in Japanese mid-fi components where there's a "straight" or "direct" button that bypasses the tone controls they still incorporate.  Undoubtedly there's a market for a preamp with a variable loudness control, though obviously not to everyone's taste.