Apple has today, 5 August, released standalone updaters for the last two Security Updates, which were first released via Software Update on 21 July:

• Mojave Security Update 2021-005 is available from here, and is 1.79 GB.
• Catalina Security Update 2021-004 is available from here, and is 1.44 GB.

If you’re looking for standalone updaters for Big Sur 11.5 or 11.5.1, then I’m afraid that you’re out of luck still, as Apple hasn’t been releasing any standalone updaters for Big Sur. The choice is Software Update for a delta update, or the App Store (etc.) for a full installer. If you’re unhappy with that, please let Apple know.

Thanks to Mr Macintosh for alerting me of these.