In April 2024 Microsoft released a new Planner app in Microsoft Teams for Office 365 E1/E3/F3, Microsoft 365 E3/E5/F1/F3 and Microsoft 365 Business Basic/Standard/Premium customers. At that time, Project Plan 1 was renamed Planner Plan 1 and includes the new Planner as well as further, premium project management features.
This month, October 2024, the Product Terms is updated for the renaming of Project Plan 3 to Planner and Project Plan 3, and Project Plan 5 to Planner and Project Plan 5.
Compare the different project management offerings here: https://bit.ly/40gTmv4, and find a useful FAQ on the changes here: https://bit.ly/4fe1WyZ.