It used to be possible to downgrade with arm11 kernel exploits, but it's not anymore. In the 11.0 update, Nintendo added a list that's stored in arm9 that prevents files from a lower firmware from being installed, and the only way around the list is by exploiting the arm9 kernel which would make downgrading pointless. You can read more about it here: https://gbatemp.net/threads/why-the...simple-explanation-for-the-rest-of-us.441373/
That's a much cheaper and dirtier fix than I anticipated.
Rather than actually fix the ARM11 kernel exploit, Nintendo just hardcoded a blacklist of titles that may not be installed.
I guess then that legit CIAs can be installed on 11.0/11.1?