Distilled water would not cause scale. I've long suspected that reduced flow is caused by small debris in the water jacket left over from the manufacturing process. With use, the debris consolidates and reduces or stops the water flow. I once tried poking around on the outflow side fitting with a piece of thin wire. The wire wouldn't go in very far, and I didn't try to force it. The flow returned to normal. If there had been scale, the wire wouldn't have dislodged it. Consolidated debris, which would probably tend to accumulate at the outflow, should dislodge easily, and in my case, did. I don't have x-ray vision, so I am left to draw inferences from what I can observe.
I'm going to continue heeding product warnings. I have better things to do than trying to prove to myself manufacturers give meaningless warnings, or don't know what they are talking about.
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