FYI: The option to turn on/off an email confirmation when an email address changes via an experience site also applies to when a username is changed (https://help.salesforce.com/articleView?id=000324558&type=1&mode=1)

While the EMAIL changed email is customizable, the USERNAME changed email is not. Seems crazy, especially when the username change is coming from an Experience site, but did confirm with Salesforce Support.

We won't be turning on the email change feature until we can customize both templates (or separate the logic). I did create an idea for this, if this is something you'd also like, please vote!

https://trailblazer.salesforce.com/ideaView?id=0874V000000lgbUQAQ