The release date of macOS Catalina might’ve been revealed through a marketing webpage that mentions Apple Arcade.
According to Juli Clover for MacRumors, Apple’s Danish page for macOS Catalina hints at a release date of at least October 4th:
“Exceptional gameplay. Find it in the App Store from October 4,” reads the translated version of the site. No additional dates are provided on the Apple Arcade specific site, with the October 4 mention limited to the Catalina page.
Google Translate is surprisingly accurate in this case.