Cabal Gold is a required material in Season 13; not only do you need it to earn Hammer of Proving charges to decrypt specific Umbral Engrams, but a charged Hammer is required in order to smash the final caches in Battleground activities for the rewards. The only way to charge the Hammer is by infusing it with Cabal Gold - so it's essential for this season's activities!
As you enhance your Hammer of Proving, you are able to increase the amount of Cabal Gold you can hold. The tiers are as follows:
Base/no upgrades: 42 Gold
Cabal Gold I: 56 Gold
Cabal Gold II: 70 Gold
Cabal Gold III: 84 Gold
You must own the Beyond Light DLC and the Season of the Chosen Season Pass