Do UV islands scaled to 0 have anything applied to those surfaces? This is the dodgy shader setup I've done https://twitter.com/James___UK/status/1262658211934543873

I have two UV maps, one for the inner surfaces of the glass jar and one for the outer, and then with the UV islands that are irrelevant to each UV map I just scaled them to 0. Do they still have any value attached to them if I do this? I have a hard time telling from looking

Apologies if I've explained this terribly
If those islands are used to map a texture, they will map a solid color (representing one pixel of the texture).
Such technique can be used to create pixelation.

If they aren't used to map any texture, it doesn't matter.
Then I'd reccomend scaling to 0,0001 instead of 0, so they can be scaled back up if needed :).