Offline Updates for macOS 10.15 Catalina?
Sometime late yesterday or early today the 10.15.1 update was made available for download (along with corresponding 10.14 Mojave Security Update 2019-001 and 10.13 High Sierra Security Update 2019-006 updates; 10.12 Sierra is likely retired now).

The 10.15 Supplemental Updates were never made available in this way for some reason, but the 10.15.1 update supersedes those updates anyhow. It remains to be seen if other 10.15 updates like the Security or Supplemental updates will continue to be available as they were in the past.