I had some thought about it too, the problem is a bit like the one I had with population variation in the mod making the "turns before growth" information something unreliable.
The UI will say 10 turns to make this, while it may takes 30 or 50 if the supply of materiel is lowered.
Well, the civ6 UI is a mess in this regard anyway. If you open the build list to see turns to complete some item, and then change the yield focus or manually assign citizens, it doesn't update the list until you close and reopen. The city banner, however, does update immediately. So you have to look in 2 places and do a lot of pointless clicking just to run "what if" scenarios. I really hope they plan to improve that at some point.
That aside, I think it would be reasonable for the player to get comfortable with the displayed build time as being an optimal number... based upon adequate supply. If there was a flag displayed in the city banner (
) that indicated sub-optimal production, I think that would be preferable to knocking the build offline, even at risk of wasting a few turns before noticing. I suppose if supply is completely exhausted and not replenishing at all, it might make sense to poke the player. Can you do a notifier drop?
I see now why you were concerned about hassling the player with too much micro from turn by turn supply consumption... it can be touchy and difficult to predict, in certain situations. Hopefully that stabilizes with the new materials buildings.
Looking at civ6 cityBannerManager... is it problematic to update the display with a modified number? Such as... add to prodTurnsLeft, a value which is the product of itself and a percentage (derived from material shortage). I'm naive here, I've not experimented with it at all
. Even if it's possible, I could see where it might not be that useful if supply is in flux.