Per my knowledge, this is a designed SharePoint procedure, it cannot be changed by users. So, custom properties for SharePoint files cannot be removed in the Microsoft word.
Every file be saved from Microsoft word into document library should in keeping with settings in the SharePoint document library. If you set required property in the SharePoint document library, the required property should be filled in the Microsoft word.
For the issue that cannot choose term in the managed metadata column, check the term is available for tagging.*************************************************************************
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