https://www.theregister.co.uk/2019/12/13/ms_pushes_team_changes_let_everyone_play/

Microsoft has been alerting admins today to changes to its Microsoft Teams Exploratory "experience", which effectively allow users to sign up for a trial rather than requiring admins to grant the privileges.

A Message Center post explained that the MS Teams Commercial Cloud Trial is being replaced with "MS Teams Exploratory experience" which: "allows existing Azure Active Directory users in your organization who are not licensed for Microsoft Teams to initiate a trial of the product."