The “Text Customizations applies only to New Universal Login” error happens if you have override the New ULP experience by enabling the Classic Universal Login custom HTML page templates. These templates include the Login, Password Reset, MFA pages.
If you’ve disabled these toggles but are still encountering this issue, I recommend referring to the knowledge solutions below for more help:
Thank you.
This information led me to the solution, which was a mis-match between the Login Flow (Applications>[Application Name]>Organisations and the template I was editing for the Custom Text under the New Universal Login.
I switched to the combination of “Prompt for Credentials” and the Login-id template and the changes are visible and working well.