Nirvana bootleg recordings was created in 1988.
Bootleg recordings are illegal recordings, so yes it is illegal.
While it is illegal to buy, sell or distribute bootleg recordings, it is not illegal to own them, for collecting purposes.
Bootleg - song - was created in 1969.
Bootleg Detroit was created in 1994.
Ghetto Bootleg was created in 2005.
Bootleg Retrospective was created in 1977.
Bootleg -Feedbacker- was created on 2005-01-21.
Bootleg Versions was created on 1996-11-26.
Rude Bootleg was created on 1986-08-24.
Bootleg - Tempest album - was created in 1991.
Official Bootleg CD was created in 2004.
The song is in fact not by Nirvana, but by the Doors. Nirvana once covered it during one of their live shows (the recording of which is what I assume you are referring too). The recording is in a fairly low quality and does not appear on any of their official albums. It can instead be found on various bootleg records, perhaps most notably on the bootleg: "Outcesticide II: The Needle and the Damage Done"