Pfizer/BioNTech and Moderna both trialed a Delta-targeted booster, but realistically a third dose of the existing vaccine was fairly effective against Delta, and the trials were overtaken by events.

Omicron-targeting boosters are currently being trialed, and might see the light of day if Omicron hangs around for a bit.

If the next Big Variant had been closely related to Delta (as might have not unreasonably been expected), the Delta-targeting booster might have been a bigger deal; as it is, Omicron is quite different. If whatever comes next happens to be Delta-related, the Delta-targeting boosters will presumably be revived.