I'm not sure that technically their can't aren't still in use...
Worship of Belisarda Entry in numerous books wrote:
With the destruction of the other four Lords, only Belisarda remains to shepherd the elorii race, teaching it the difficult lessons of patience and forgiveness. Priests still maintain the sacred fires in the temples to Keleos, the cool pools of the temples to Beröe as well as the other two, but those gods remain silent and grant neither gift nor counsel. It is Belisarda who allows these priests to channel divine power to power their rituals.
It could be that Belisarda's cants have been taught to the priests of the other 4 elemental gods ... or it could be that most of their Cants are empowered by Belisarda (Belisarda obviously can't empower the Deity Domain spells of another God but theoretically ought to be able to facilitate shared Benedictions, Corpus & Wards from their combined Pantheon).
Just a guess. Its an interesting question.