Keychain Access app opens on login Big Sur. Did not set as login item
Solution 1:
When you have log in and see Keychain running, try right-clicking its icon in the dock. Under "Options", you'll find "Open at Login" - make sure it is not checked.
Then right-click the dock icon again and select "Quit" to ensure that the application is fully closed.
Now you can log out and login again, and hopefully Keychain Access won't start up automatically.