They were not destroyed by Agathocles. The army they were in retreated and they retreated as well. Sources don't speak about a "sacred band" but more of the wealthiest of the Carthaginians with the best armours and weapons. There isn't anything supporting their continued existence as a unit because we only have a few Roman and Greek sources on Carthaginian military history. But since this unit was composed of the wealthiest Carthaginian citizens you can still use it for roleplaying in Africa and Sicily since Carthaginians continued to fight on wars after Agathocles, especially against Regulus invasion of Africa (two battles), they were also probably used during that time to fight the Libyans/Numidians since the writings speak about those peoples attacking the Carthaginians at the same time as Regulus invasion and causing even more damage to the Carthaginians.
After that, we have Carthaginians fighting during the mercenary revolt (various battles), at Zama and later, at the siege of Carthage, all citizens were on arms. Therefore, Carthaginian citizens continued to fight in wars after Agathocles and while the texts don't mention a "sacred band" type of unit present, we can presume that some of the wealthiest Carthaginians were also fighting, and that's why this unit makes sense for Carthage.
Of course, employing it outside of Africa and Sicily is not very correct but who am I to judge? I love to use them in Iberia with the tribes