Hello,
Dimensions are available in Financials. I don't know about Operations, you would need to ask over on the Dynamics 365, Enterprise Edition forums.
You must be on user experience 'Suite' to see Dimensions in Financials. It does actually warn you of this if you attempt to migrate into Financials and have Account Segments.
You can navigate to the Company Information window to switch the User Experience to Suite if you haven't already done so.
Thanks,
Chris