Yes, it was. Our first on site stay was in 1983and it was allowed...listed as a perk...for some years.
I have never seen an official reason why it was taken away but I think it was just that so many more rooms were added to WDW when they started to build the moderate and value resorts that it was necessary to limit pool hopping so that the more exotic and/or more convenient (to the parks) pools weren't over run by all the thousands of extra on site guests.