Apple has released macOS Big Sur 11.6 and Safari 15. Both can be downloaded by opening System Preferences and choosing Software Update.
I’m not sure what changes are in macOS Big Sur 11.6 aside from a new version of the web browser. And, according to Apple’s release notes, Safari 15 delivers faster performance, improved security, and the following new features:
° Tab Groups help you save and organize your tabs and easily access them across devices
° Redesigned tabs have a rounder and more defined appearance and take on the color of the webpage
° Compact tab bar option shows more of your web page on screen
° HTTPS upgrade automatically switches sites from HTTP to more secure HTTPS when available