CSST is perfectly acceptable by most model codes. If it was acceptable at the time of construction, it’s still acceptable unless your local jurisdiction has prohibited its use. A call to the Building & Safety Department will clear it up quickly.
Codes are not retroactive (that’s the concept behind “grandfathering”).
As Bob eluded to, there are additional bonding requirements when CSST is installed. If you are in a high lightning zone (LDZ - Lightning Density Zone) you may need additional bonding of the gas lines to the residence grounding electrode.