Probably a lot, but what comes to mind right off for dead (essentially) groups... not sure how many people involved are literally dead:
Broadcast
They're not dead yet I hope! But they are not exactly prolific and haven't put anything out since their full length debut a couple of years ago. The shows include psychedelic-like stage projections befitting their cinematic sound.
Can
Kraut-rock fun... would have liked to see them with Damo Suzuki on vocals.
Kraftwerk
up to and including Computer World
X-Ray Specs
major punkers led by proto riot grrrl Poly Styrene
Dead Can Dance
how do you describe them... ambient world-goth?
Buzzcocks
the original edition
Wire
likewise... though they're sounding not bad at all in their current reincarnation
Joy Division
I DID get to see New Order during the Lies tour
My Bloody Valentine
after Loveless, of course... and with ear plugs
Funkadelic
yes
Cocteau Twins
no later than Blue Bell Knoll
XTC
before Skylarking when they were interesting
Nico
a friend saw her in the 80's at a small club and I regret not going
electro-acoustic performances
I don't know whether Pierre Henry or Morton Subotnick performed live... saw Jon Appleton once.
various jazz greats
Coltrane, Davis, Reinhardt, Baker, Parker, Sun Ra, Monk, Modern Jazz Quartet...
yikes... I'll stop
