As Apple moves toward Apple Silicon and macOS 15, the window for Catalina is closing. By late 2025, Apple will likely purge Build 19A583 from its content delivery networks entirely.
You might wonder why someone would want the very first public build (19A583) rather than the final Catalina update (10.15.7, build 19H15). Reasons include: macos catalina 1015 19a583 mac app store dow patched