AH ha, I found it!
Wood beam freight cars were banned in interchange service in 1928.
The ICC banned billboard type advertisements on cars 1937
Arch-bar trucks were banned in interchange service in 1939.
By 1900 all cars had to have “modern” safety appliances, air brakes, knuckle couplers, and proper type and numbers of grab irons.
But of course this was for interchanged cars. Cars that stayed on their home road didn’t have to comply. So many of the Ely Thomas (and other) log cars of the 1940’s and 50’s had arch-bar trucks. I guess they were cheep on the used marked because they couldn’t be used. Several railroads had wood framed cabees in service into the 1950’s, 60’s and some even later. Link and pin couplers continued to be used on mine and logging trains long after 1900, and many of those types of cars also didn’t have air-brakes.