I have an old 75g tank without a center brace. Stock, assembly line tank from the factory. Probably made with thicker glass, as already stated.
These center brace-less 75g tanks are prime to use for a sump. I can take a pic of it if you'd like to see.
If you have a 75g tank that currently has a center brace, I would not recommend removing it. Unless you plan to remove the whole top brace and do euro bracing.