I just noticed that in the release notes for Safari 13.0.3 beta 1, it notes that the beta is available for High Sierra and Mojave, as well as Catalina 10.15.1 Beta, which obviously doesn't exist yet - so it must be coming very soon ?
Don't have you expectations high boys, it will take at least half a year (and more than one beta) before Catalina becomes as usable as Mojave currently is.