How to disable startup security on MacBook Air 2018

Hi!

Is there anyone who succeeded in disabling the "secure boot"/"startup security" option on MacBook Air 2018 and boot any OS different from macOS or windows 10 from an external drive?

I was trying to follow this instruction from apple: https://support.apple.com/en-ca/HT20833

but it didn't give any apparent success. I was trying to start windows 7 or linux from an external drive, and I saw exactly the same behavior as with secure boot enabled: no screens from windows/linux at all, not even fatal errors (BSoD/kernel panic), only from mac. It suggests to update something, then fails, and does not boot anything else than macOS.