Hi @Root Loop ,
Thanks for posting here.
Below is my response to your questions. Frankly speaking, I am not professional with this issue since I mainly focus on on-premises AD issue. I did some research and hope the findings could be of some help to you.
- IDMU and NIS will not be supported starting with Windows server 2016. So if we would like to upgrade all our domain controllers to Windows server 2019, IDMU and NIS should be removed.
In our case, there is no server in the domain to assign as the master. We recommend to start planning for alternatives, for example: native LDAP, Samba Client, Kerberos or other non-Microsoft options.
- Sorry that I have no idea what will happen to NIS domain. Maybe as discussed above, we will need to look for alternatives.
- If we have concerns, I would suggest you open a case with MS so that we may get a more professional assistance.
https://support.serviceshub.microsoft.com/supportforbusiness - Below are the links I would like to share with you.
Thanks a lot for your understanding and support.
Best regards,
Hannah Xiong