It has to be the last beta because public beta testers were forced to upgrade to 13.1 beta 1. I don’t see how they could release the GM to beta testers when 13.1 is higher up than 13.0. Unless the public will end up getting 13.0, with all the bugs with the mail app. I’m sure that will go well.
Believe it or not, Apple can still "guide" your device down an upgrade path without a profile. They could release a GM that only devices with a beta iOS 13 build (letter at the end of build) could see (without the beta profile), which could be how they roll out the GM. Or they may force users to install it via IPSW.
Obviously retaining the beta profile means you'll see the latest iOS 13.1 beta.