When a user tries to access a Site, they are redirected to id.atlassian.com where they have to key in their email and if SSO is enabled, the user then gets redirected to the IDP for Authentication.

Give an option to the Site Admin to enforce a domain hint so when a user tries to access a Site a domain hint like below gets appended to the request so that the user does not have to type their email on id.atlassian.com for SP initiated logins.

https://id.atlassian.com/login?application=statuspage&continue=https://manage.statuspage.io/auth/atlassian?origin=%2Fcloud%2F18094dba-5e8f-4db4-baeb-2a31024c49ba&domain_hint=domain.com

Points of Concern:
In the above case Org Admins will not be able to use the Authentication Policy Feature as all users will get an Enforced SSO login.
Users from other domain will not be able to join the Sites.