Time Machine Server has not been removed. It has just been moved. (And well hidden) Do the following:
1: Open System Preferences > Sharing
2: Define the shared folder that will be (or already is) your TM folder
3: Right click on the folder in the Shared Folders list (or hold down the Control key and click on the folder) to get the contextual menu
4: Choose Advanced Options...
5: Check the box "Share as a Time Machine backup destination" and optionally define a limit
That should get you back and operational.
But, and this is a little fuzzy right now, you might need to reconfigure the clients. (We are all figuring this out). If your server has an SSD drive, it converted to APFS. If your TM share is on the boot volume, then it is now only available over SMB. If your clients are configured for AFP, you will need to update your clients. If your TM share is on an external volume than you should be fine as external volumes should not convert to APFS.
Hope this helps
Reid
Apple Consultants Network
Author - "El Capitan Server – Foundation Services"
Author - "El Capitan Server – Control & Collaboration"
Author - "El Capitan Server – Advanced Services"
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