There is no read-only feature in Jira Data Center or Server.
When customers do a phased migration with Jira Cloud Migration Assistant, they need to perform a lot of manual steps to ensure end users are not modifying the data while a migration is happening

• Lock down projects on Jira Server / Data Center.
• This is currently done via remapping the permission scheme of all migrating projects with a new permission scheme
• Run the migration using Jira Cloud Migration Assistant
• Post Migration - Remap the permission schemes of the migrated projects in the destination cloud site, to their original values before the migration.
• Even for a migration with only 300 projects, customers easily spend about 2 hours just mapping and re-mapping permission schemes.