iOS 13.1 will be available as a Day Zero Update for all new iPhones. iOS 13 beta 8 will be the version that is installed on new iPhones on launch day, and you will have to update your phone to iOS 13.1 when activating it.
iOS 13.1 will not be released as a zero day update to the public on the same day iOS 13 is released. I agree we may see a zero day update when iOS 13 is released, but it will be either in the form of 13.0.1, or the actual GM of 13.0.0 (which is what's being loaded onto the new iPhones now) will be a different build from beta 8. Keep in mind Apple is internally many builds ahead of what is released even to developers.
It’s highly likely that Apple will make the latest dev beta available for iPhone 11 devices ON launch day just like they did with iPhone X back in November 2017
https://www.macrumors.com/2017/11/03/apple-releases-ios-11-2-beta-2-iphone-x/
Many of hope so, but Apple isn't obligated to do this.
Fyi regarding beta 8 being Gm: Apple has released a beta as a GM before. I remember times where I ran e.g. beta 9 of a release and the GM did not show up for me because it was the same build as b9. So it can very well be that b8 is this years GM.
What‘s new is the fact that they pushed out the .1 beta before the keynote / GM is available. That‘s a first.
The GM never has a letter attached to the end of the build number, as a letter attached to the end of the build indicates it's a beta. I don't recall Apple releasing betas without a letter at the end of the build number, but if they have, then it was the GM/public release disguised (and likely by mistake).
Apple could of course drop the letter from the beta 8 build, and that could then be the GM build, but in that case, your device would see this an a software update and prompt you to update OTA. GM/public release also strips the feedback app as well as logging files. In summary, beta 8 is not the exact same software as the GM, but I agree it could be more or less a similar build.