Hi Martin,
For your question, I wan to confirm:
1. Is there any other server role in Exchange server?
2. Does this mailbox server or mailbox database copies in this server has been offline over a year?
As Edward mentioned, it's recommend to build other DAG members to achieve high available.
Furthermore, please remove database copies, and this useless member server from DAG before uninstall it.
Note that you can only remove DAG members of a healthy DAG.
Allen Wang
TechNet Community Support
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