Defender for Endpoint for servers can still be purchased from resellers as a stand-alone per-server license. The tool and documentation are directing customers to the new method.
You can activate Defender for Servers P1/P2 which includes Defender for Endpoint. You get both bundled together. This also eliminates the need to purchase or maintain a license. The Defender for Servers charge is part of the subscription bill and is only billed for active systems. Meaning you only pay for active system (no license overallocation) and you don't pay for VMs while they are deallocated.