Add a default login domain to the Company Branding Page
In education specifically, students in earlier grades can have difficulty with entering their full UPN.
In our case, we have our primary domain - school.org for all staff and a student subdomain - students.school.org. If we were able to specify a default authentication domain like students.school.org within our company branding settings, this would allow students to sign in with the first half of the UPN only.
We struggle with student logins to federated applications purely because of the domain portion of the UPN. Also, coming from using ADFS in the past, students never had to enter their full UPN in that scenario.
These login issues disrupt classes and take away from teaching time in order to resolve the problems for the students.