I just had to do this in our JIRA instance. You can create a user group or role and set project permissions for each project to only allow read only functionality. Not sure if you're using different permission schemes per project, but if not, should be able to follow these steps to do it: https://confluence.atlassian.com/jirakb/jira-cloud-how-to-create-a-read-only-user-779160729.html
Just make sure those users in said Read Only group are not in any other JIRA group (as it may grant them permissions to edit, based on settings).
Please check the permissions: https://confluence.atlassian.com/adminjiraserver073/managing-project-permissions-861253293.html
I guess you need to grant Browse Projects permission and restrict issues related permissions.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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