The straight line extrapolation trick. 
I wasn’t around when this happened: Discussion re: Ingot drop rates, June 2018, but the thing with deeds and books… as and when the devs decide.
Deeds were added Aug 2019 (?), and Epic tasks to gain them were added Dec 2019. It’s more constrained than the ingot-flood from delves, but comparing the two, the ingot economy is probably the one with the wrong model for long term end-game content.