What happened to the loudness control?

I wish I still had one since the Fletcher Munson curves still apply in all cases. Try McIntosh preamps - they allow the user to control whatever they want or if you so wish, leave the controls out of the circuit, whichever you prefer.

speaker wire isand interconnects are today's version of tone controls and loudness buttons. its ironic that the audiophile community has a distain for them, when they are a far more cost effective way to adjust a system to its room conditions than cables and room tweeks. we hear they were abandoned by many manufacturers, because they changed the sonic character of what the recording (no kidding).....as if everything else we do does not. the fact is, the fewer convenience features on a preamp dramatically reduce the cost of any properly made, high quality piece.....get ready to spend a bunch though to try and compensate each 'recording's shortcomings' with lots of other stuff.

Such intolerance for tone controls! Who makes equipment that keeps the acoustic impact equalized across the entire power spectrum?

If I'm listening at low enough levels to want a loudness button, the sound is compromised anyway and the button would improve my listening experience. I would be nice if it was out of the signal path when off.

No, I do not miss the "loudness" button nor do I miss the tone controls in my ref system.
(I do have both in my HT & bedroom AV receivers! My HT receiver has a "direct" button allowing me to bypass loudness, bass & treble boosts).

Loudness button is just a bass boost button that increases bass by 6-9dB. It's a filter. Tone controls are filters as well.
Both colour the sound while providing the function intended. If one considers purity of the signal then these control compromise it. You can use the best parts to create these controls but it WILL alter the original music signal.
The approach taken in higher end audio equipment was/is to use the shortest path between input & output while still providing the required function (preamp or power amp or D/A or step-up, etc). Has higher end audio acheived this goal? Subject of endless debate!! :-)
Less signal processing => less compromised sound & a better chance that you will be hearing the recording rather than the electronics.

The "purest" of audiophiles, the vinyl fans, think nothing of the RIAA equalization (drastic) that is applied to their signal. And yet the minor adjustments needed to restore tonal balance at low volume is viewed with horror. A loudness control can hardly be called "processing". It requires no additional amplification stage, and can be switched out if you prefer.