I heard about this too and there's a large discussion about using AGM batteries as an alternative, or even a standard battery but from what I read a standard battery dies rather quick (~2 years). Oddly when searching on Costco, for a '16 Mazda 6 with i-Eloop tech it list both a standard and AGM battery as options too. Have you tried either?The old iStop mandates a Q85 expensive 12V battery.
Hope the *new* iStop does not.
My '16 Mazda6 GT has the iEloop tech (supercap to drive accessories), which also came with a Q85 12V battery. It costs up side of $400 to replace it every 3 yrs or so (mine failed after 3 yrs).