Yellow-jacketed CSST (corrugated stainless steel tubing used for natural gas) must also be bonded. This was added to the code in the mid-2000s so homes built from the mid-nineties when CSST became available till around 2005 when bonding requirements for it were added may not have it. I had to retrofit it to my 2004 home.
https://structuretech1.com/csst-bonding-requirements-final-u...
http://csstsafety.com/csst-faqs.html