If a customer had Jira Software installed on a site and removed it, the Issue Type Hierarchy menu will be available to the JSM product however the admin will be asked to install the Jira Software with premium version to create additional levels. The user is also not able to assign the existing level 1 to an issue type.

Please consider removing the Issue Type Hierarchy page when the Jira Software product is uninstalled from the site and make the Issue Type Hierarchy unavailable for JSM since as mentioned in the documentation:
https://support.atlassian.com/jira-cloud-administration/docs/configure-the-issue-type-hierarchy/
https://support.atlassian.com/jira-software-cloud/docs/configure-custom-hierarchy-levels-in-advanced-roadmaps/
This seems to be an originally Jira Software designed feature, used with Advanced planning for Jira Software.