This is interesting. AAD Connect seems to allow us to continue with Hybrid Join configuration, though we opted to not, until this is clear. Microsoft support has verbally told us it should work, but couldn't provide any documentation confirming it. And now NagappanVeerappan-MSFT seems to be saying the same.
Meanwhile, what's also interesting is the use of UPN and Join, here. As we all know, UPN is user principle name, and should refer to user accounts. As far as I know, computer account objects do not have a UPN; ASDI does not show any field with a UPN value that matches the routable UPN suffix we added. Also, the term Join is a computer object term, not a user term; computer accounts join a domain.
So, it's interesting to me how that online doc https://learn.microsoft.com/en-us/azure/active-directory/devices/hybrid-azuread-join-plan uses UPN at all, in relation to a discussion on Hybrid Join. As stated, our domain has a routable UPN that we added for user accounts, and it matches their Office 365/Azure Tenant, and we sync our user AD on-prem accounts including passwords.
Why is this so hard to get clear documentation on this issue, a guide on how to get from non-routable domain AD to Azure AD, computers and users etc. If your goal is to get to Azure only environment, Federation isn't it. that makes the situation worse by making all authentication dependent upon on on prem system.