Is that still a thing? When does it kick in? Is it indicated anywhere in the Advisor page?

nope nowhere on advisor, best I can tell you is once you reach 1m bushels food decays at 10%
2m bushels 20%, 3m bushels 30% so if you reach more then 10m bushels you are at 100% decay.

so always keep 999,999 bushels! :p

Or send it on the market, let's tell him guys....