First a definition - as opposed to piracy, a bootleg is a recording that a label isn't selling - like a radio broadcast of a concert or unreleased studio demos or outtakes.
The first bootleg was a Bob Dylan release - Great White Wonder. It contained unreleased studio outtakes and took the music world by storm.
The bootleg sold out and other labels appeared with stuff from The Who, the Stones, Led Zeppelin and other bands.
Tales From The Who is my favorite cover.
In the 1980's and 1990's, cd bootlegs came out. Eventually websites offering bootleg only music torrents arrived. The ones that survived are actually strict on piracy - if a concert has one song that was released, that song is not included.
Printed silver disc bootleg cd's were expensive - I paid $25-$30 for most of mine. Insane! Some are worth up to $500 each. $100 for a good Led Zeppelin 3 disc concert is a normal price.
That scene grew until a few years ago. If you look at the bootleg torrent sites, they are a snapshot of early PHP websites.
What happened?!
Many collectors moved on - after 30 years of music collecting, you may be done.
Many are older - some of the people I met online way back when are now in their 60's.
And the big reason -
Bootlegs have evolved into audience recorded YouTube clips. Every major concert will have a few or many clips. The Who and Paul McCartney's Facebook pages even post audience clips after a show!!
Peter Grant, the legendary Led Zeppelin manager, beat the &^%# out of tapers at concerts in the 1970's. Today, the industry has embraced reality (finally). Those audience recordings on YouTube are souvenirs of a show.
The young people growing up today aren't buying cd's or physical media (yes, vinyl is a nice niche), so they will never buy a silver disc bootleg.
Check out Led Zeppelin's Earl's Court show on YouTube, and The Who's Tales From The Who!
What happened to the music bootlegs? They ended up a normal thing to do and watch on YouTube.
Enjoy the music!