Apple distributes the new Safari 15 version via the software update to macOS 11.
Besides the regular security updates, you will also enjoy a redesigned browser tab design. Also, the color scheme of the browser is now dynamic: it is based on the main colors of the accessed website.
Some will like this very much, some will not like it so much. There is a solution for all of them: if you like it, leave everything as it is. If you want to restrict the changes again, you can do that in the settings under “Tabs”.