<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=562419) </details> <!--IssueSummary end--> In https://gitlab.com/gitlab-org/gitlab/-/issues/525342+, importers that mapped contributions to the Import User bot in personal namespaces were updated to assign all contributions to the personal namespace owner. Once `user_mapping_to_personal_namespace_owner` [has been rolled out](https://gitlab.com/gitlab-org/gitlab/-/issues/556557) and existing personal namespace source users [have been deleted](https://gitlab.com/gitlab-org/gitlab/-/issues/534710), the `Import::SourceUser` model should validate that namespaces are not personal namespaces (`namespace.user_namespace?`)