Thickness has to be judged together with width
A narrow, thick shank doesn’t behave like a wide, thin one. Across about 75% of the shank, from the bottom upward, my personal lower limit for thickness is about 1.5 mm.
And what about shank width?
My personal lower limit for width is about 1.4 mm - and even then only when the thickness in that area is around 1.6–1.7 mm. These are not universal recommended dimensions but points below which I stop and re-examine the structure.
Why isn’t there one number for the whole piece?
The bottom of a ring, a shoulder, a head, a prong, a frame or a decorative element don’t do the same job. The metal type, how the piece will be worn, stone sizes and proportions all play a part as well.
A minimum is not a target
Being able to produce a detail at a certain thickness doesn’t mean that is the thickness to choose. When a client asks to go below the level I consider safe, I explain what it means and offer an alternative; going ahead against the recommendation should be an informed decision.