This username field is to identify the user source and navigate them to the IDP they are originating from in case they are not sure which IDP to select. During the SSO setup you will identify the source and populate the IDP name in one of the user attributes (CP3) and the same attribute is selected under the Multiple IDP lookup fields during the SSO configuration. Hiding this field will defeat the purpose.
Let's say there is multiple IDP setup in your organization with both PING and OKTA with some business requirement eg., Full-time employees are in OKTA and Contractors are from PING. Now let's say you are a full-time employee, your administrator will update the cp3 value as okta. Then when you access the login page and provide your username, you will be automatically navigated to the okta login page without having to select the IDP from the below dropdown.
Hope I have clarified your question. I am in process of creating a knowledge base article around this with a recording and will share it here once it is completed so that it may provide you more clarity on the usage.
We already had this functionality tested before posting this on Forums. We currently are running out of custom property fields and hence we do not want to enable this functionality. So if this is left out there with no config defined, it will confuse the end users, hence we wanted to see if we can hide this.
Also is there a way we can have customized message displayed on the login page? As you told for eg. Full-time employees are in OKTA and Contractors are from PING. We wanted to display this on the login page. Is this possible?
It's not possible to hide or make any custom changes from UI. However, as a workaround, you can change the IDP name from Okta to Okta - For Fulltime employees in order to provide directions to the end users
