I came across this on my inspection yesterday where the gas line to the water heater was converted from Black steel to a copper pipe. I have not seen this before and I was trying to look up my notes and do a search on the forum but couldn’t find something recent so I apologize if it was mentioned already.
That pdf on copper is not your usual dry information flier.
Copper tubing for gas seems to be a regional thing. It is more common in some places that others. I rarely see it. You might comment that there is no evidence that copper piping is rated for this use or used appropriately.
Copper (or even galvanized steel) is allowed in Oregon for the most part. One interesting rule I came across is that it’s required to have yellow stickers that say, “gas” on the pipes that are routed through areas outside the room where the appliance that is served. I guess that’s to keep a plumber from cutting it open thinking it’s water pipe (and they assume he’s smart enough to trace the pipe to the appliance if it’s in the same room).
It’s super rare to find anything other than black iron or flex pipe but every once in a while I’ll see it. Most common is probably a copper line from a propane tank
Copper is everywhere in Minnesota… likely due to the high use of LP Gas in the mostly Rural state!! Some areas require that the copper is “coated”, although I can’t visually see any difference, or any markings, on the “tubing”.