Is Sterak's Gage the staple ADC defensive item now?

I've never seen an ADC build Sterak's.

But, if it were an issue, they could just give it a "Melee Only" tag, like the Hydras.

Keep your ADCs away from my items.

The base AD bonus is good for Triforce users, but the shield is based on max health so not as effective on a non-tank. Not sure it's quite good enough.

Probably not. Your HP wont be high enough for a strong shield. And unless you proc it for the extra damage, you're not getting much else out of it other than the HP.

It could do some crutch saves, but I don't think it's powerful enough or an ADC.

1800 health is pretty much 75% of an adc's health

Nope.

If it was confusing, Base AD means the AD your champion gets naturally from leveling (not the green number) which for most adcs, is around 25~ AD. The bonus from shielding isn't really worth since it'd be around 600~. It might save you, but so would Spirit Visage, Randuin's Omen, Deadman's Plate, Guardian Angel, or especially Banshee's Veil. It's an option, but it isn't the best option for ADCs.