An open source project disappeared from SonarCloud on its own

[Colin]

Hey there.

We introduced a housekeeping policy at the end of last year for public projects on accounts with no private projects.

New housekeeping policy for public projects

Dear SonarCloud users, Please note that we’re putting a new housekeeping policy into place for public projects. This policy affects all public projects that have not run an analysis for over a year, belonging to an organization on a free plan. These public projects will be removed from SonarCloud. Organization and project members will be notified in the web app before the removal of a public project. For more information about this policy, please refer to our housekeeping documentation page. …

You should be able to create the project new from within SonarCloud (and can even keep the same project key).