Specific case history as an example for the young guys:
Grandpa built his house in 1906, all gas lighting, no electricity available.
1914, electricity became available (central IL), and grandpa wired up the house - all of 2 (two!) outlets and 5 ceiling and 4 wall lighting fixtures, plus newfangled electric range that had it's own outlet.
ALL the lights were wired pulling linen wires thru the black iron gas pipe, elbows cut out and K&T at the bends. Main disconnect consisted of 1 open knife switch behind a wooden cabinet door and two small 3" by 5" ceramic blocks with pieces of solder for fuses. Still have one of the fuse blocks in 'historic' collection of parts - mPc part number 1183/4.
Gas in 1910 era was NOT natural gas, it was coal gas derived from making coke for the steel mills over in Indiana. When house was converted to electric lighting, the gas main was disconnected.
No changes till late 40's, when pop put in a sq D panel and added lights and outlets in the basement. When coal furnace was converted to nat gas in early 1950's as nat gas came to central IL, most, but not all, the gas lines from the street were replaced, gas meters installed, 100% new piping for gas furnace, etc., so zero chance of gas in any of the old black iron pipe.
Grandma died in 1975, the light fixtures still had the gas jets, but no gas. Pop used the house for storage until 2003, gas light fixtures still there then, installed 200A service a few years ago and brother rewired entire house after Pop's death, but left some of the ceiling fixture gas jets in place for 'ambience'.