The Absence Submission object and the associated Absence Schedule record types on the Event object are packaged items. However, the user interfaces for the creation of Absence Submissions from within a Salesforce org or from within an Experience Cloud portal are delivered as unmanaged components.
An Absence Submission API has been provide to facilitate the creation of Absence Submission and Absence Schedule records in a controlled manner. However, clients are free to customise the user interfaces of the Absence Submission entry windows as they see fit.
API call information can be found here: https://alphasys.atlassian.net/browse/E2-340