Did they change it so you need to be in the Apple Developer Program to access the latest Xcode beta version or GM Seed? Because I remember downloading Xcode Beta not too long ago. It says on https://developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/WhatsNewXcode/xcode_9/xcode_9.html :
Prerelease versions of Xcode are made available from developer.apple.com to authorized seed developers, packaged in a compressed XIP file.
Does seed developer equals being a member in the Apple Developer Program?