@dan, Thanks for posting in Q&A. Based on my researching, windows 10 or higher is supported to do Hybrid Azure AD join. But it is recommended to upgrade to latest version as windows 10 21H2 previous version is end of support.
https://learn.microsoft.com/en-us/azure/active-directory/devices/concept-azure-ad-join-hybrid
We can follow the documents as below to plan and configure:
https://learn.microsoft.com/en-us/azure/active-directory/devices/howto-hybrid-azure-ad-join
however, if the Hybrid Azure Ad join is failed, you can troubleshoot it according to the following article:
Hope the above information can help.
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