Yes, the old deities are really coming back.
Along with the list of old deities in the forgotten realms section of the gods listed in the updated basic rules, (and the gods listed among-st many pantheon in the player's handbook) there is also the "Sundering" story line. The Player's handbook lists 39 divine for the forgotten realms setting with their domains, and symbols. And 24 "non-human deities" which are shared between Greyhawk and Forgotten realms.
During the Sundering, the worlds of Toril and Aibil which were united during the times of upheavel, are being separated from each-other. During this time, the various gods are creating "chosen" mortals which are intended to bring power to the gods, in the hope that when "things are settled" these gods will have the power required to remain a god. There seems to be confusion among st the gods as to what Io actually has in mind as he re-writes things.
Noted events during the sundering are the diminishing of spell plague affects, magic changing it's nature to the way it worked in the past, such that old spell books work and recent ones do not. "The Weave" has new prominence, and most importantly for this question, various gods are found to either not be dead, to be re-united, or to be recreated through various events. It seems that in the various divine schemes to "get enough followers" all divine who have a "chosen", get chosen to remain a god, in a fun twist of the meaning of the "chosen".