I personally write up copper in a natural gas system, but it is almost always just a short piece used instead of a proper flex line.
It is a simple fix that needs to be done even if the gas is ok.
I just use a generic statement and let the plumber worry about the details.
Replacement is cheaper and easier than researching the specifics. Gas that might be ok this year might not be in the future, not worth the risk.

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002