They were replaced in 1997. Back in the 90s B-52Hs were fitted with the Common Strategic Rotary Launcher. Those planes could no longer carry the B53. The B53 role of bunker busting by laydown delivery was replaced by the earth penetrating B61-11, up to eight of which could be carried on the CSRL, instead of the single B53 previously carried.
So all the press hoopla about dismantling this thing is just Obama's press corpse (pun intended) trying to make people believe he is following through on a campaign promise, when in reality it was begun under the Bush admin.