As you mentioned, the user writeback feature that was used in the previous builds was a preview feature and was removed in 2015 due to a variety of issues with it. (I'm not sure what all of those were, but I suspect it had to do with attribute mapping and keeping the source of truth for the user attributes in one place.)
One workaround which you may have heard about is to create a PowerShell script that scans Azure AD regularly, finds the users in Azure, and then creates an on-premises user with the attributes in AAD. (There is an example of this here from Peter Stapf, if you haven't seen this yet.)
I've pushed your request to the Azure AD Connect team to see if they have an update about whether this will be supported in the future. I haven't heard of any recent plans for this, though it has been requested frequently. I will let you know what they say.