System status notice about option value 0 in number field
We finally poked around and noticed standard option groups with integer values did not use zero. We had used value zero to represent "No". We changed "No" to value 3 and updated the existing records. Once all zeroes were gone, the warning disappeared. Later, we noticed that other radio button option groups use Alphanumeric data type even if the values are integers. Ours had been configured as an Integer (INT11 in the database). I can't say whether either of these conditions are the root of the warning, but avoiding zero appears to have worked. See also:

Option Values with problematic Values