Hi @sns ,
Per my research, Custom master page files (*.master) shouldn't be migrated to the new platform. The setting on the destination site will be set to the default master page. This process ensures the site will render after the migration. If you decide to continue using the customized master page on the new platform, you will need to apply the master page setting and validate your customizations work on the new platform.
For Reference: Migration Assessment Scan: Master Pages
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