Front page of the wiki has a section for discontinued packs.
Currently only 2 packs are listed there: FTB Beta pack A and Pax Challenge Map pack.
I think the number of discontinued packs has been growing as new minecraft versions have been released.
Most modders said it's easier to compile a new pack instead of updating all the mods of the older packs (keeping the same ID numbers is a hassle, also simple waiting for 1-2 modders can take a long time). - I think this is pretty clear.
I want to discuss here which packs to move to the discontinued list, because I am not sure about the status of some of them.
Definitely to be moved:
1.5.2 Beta Pack (v0.8)
Feed The Beast Lite (v1.2.2)
Feed The Beast Retro SSP (v3) / SMP (v1)
Not sure about pack's status:
Magic World (v7.2.1)
Tech World (v1.2.1)
YogCraft Pack (v1.0.0)
Slow's Stream Pack (v1.0.0)
Feed The Beast Ultimate Pack (v1.1.2)
MindCrack Pack (v8.3.2)
Direwolf20 1.5 Pack (v1.1.3)
The 3 last ones are still used by some players, not sure if we can call them "Discontinued" (they are though), maybe creating an "Archived" section for them would be more reasonable.