Thanks for posting your question in the Microsoft Q&A forum.
When your last Project Plan 1, Project Plan 3, or Project Plan 5 subscription expires, your Project for the web instances will not be automatically deleted until you have no active subscriptions that depend on the Microsoft Dataverse.
An appropriate Project subscription (Project Plan 1, Project Plan 3, or Project Plan 5) is required for your Microsoft 365 or Office 365 users to view a Roadmap or a Project for the web plan. Users assigned with certain Microsoft 365 or Office 365 subscriptions can have limited edit access to projects shared with them without needing to be assigned a Project subscription.
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