I'm reading a book on Applied Acoustics and they mentioned different types of fiborus glass or metals as a means of passive sound absorption.

I recall way back when there used to be some models using fiberglass or steel wool packing....a la Cherry bomb mufflers.  

Why did the use of fiborous sound absorbers fall out of style?

new technology...

Durability and capability.

It would be nice if gun ranges would invest in passive sound absorption though.

My local one has some sound barriers and I can say acustical sound barriers for subsonic noises does not translate for jack on sonic booms. If anything it's just another surface for the noise to be reflected back to the shooter.

Do you mean supersonic booms? They are a high intensity waveform to deal with. Passive absorbers and  reflectors would need to account for that.  the problem is that you are probably in a shoot lane box with nearby walls and a roof covering and a solid floor or even cement (describing the 2 ranges I have here and assuming you might see something similar).  these are great reflectors and can intensify the noise.  Especially the cement o hard ground.  Imagine being in the sound box of  an acoustic guitar.

I understand technology improved so no need for the glass-packing. But there are perforated metal standoff walls that passively diminish the noise... sound waves go through the perforations and get trapped in the space between the wall and the thin metal panel.  It could have a similar affect as fiber-glass.  Things like this are used in industry like Tectum plates, or absorption panels in classrooms/theaters..