Comments from Eric Lombard...
I see no one provides definitive information about this interesting question. Perhaps because no such rule occurred.
I will take a stab at a non-definitive and inferential answer in support of that position. This claim is based on active survivors originally built or rebuilt with grabs instead of ladders and still in service after 1964. My box car database coughs up 111 series with grabs instead of ladders still active on 12-1964 (and later). These cars are clearly on their way out to the scrap yard having been originally built prior to 12-1944 but many also were rebuilt after that date but not in a way that changed the grabs.
In 10-1966 a running board and full ladders were banned on all new cars, and on 1/1/1971 cars with running boards were banned in interchange. Yes, in this period there were still a few cars with grabs still in service.
For example: In 1-1971, 30 in UP 180000-182346, B-50-17 (steel sheathed, grabs) were in service. These cars were rebuilt from 125900-127899, B-50-13 (wood DS, grabs) and 171500-172999, A-50-7 (wood DS, grabs) in 1935-36.
The B-50-13 and A-50-7 were built 1922, so in 1971 the underframes on the B-50-17 heroes were nearing 50 years old.
There are many stories like this and my data at 111 series presents an undercount since many series are yet to be traced out to their final extinction since I tend to loose interest about 1960.
Eric Lombard
Bob Chaparro