While running the vSphere 6.7 VMware Migration Assistant I received the following error:

This vCenter Server has extensions registered that cannot be upgraded to or may not work with the new vCenter Server.

In my case, the extensions were no longer being used.  So, I chose to just remove the offending extensions.

1.  Create a snapshot or backup of your vCenter server.
2.  Log into the Managed Object Browser (MOB).

https://vcenterfqdn/mob

3. Select Content.

4. Select ExtensionManager.

5. Capture the VALUE of the Extension you wish to remove.  (ex.com.vmware.vcHms):

6. Select UnregisterExtension

7. Enter the Value of the extension you wish to remove.  Then, select Invoke Method.  Confirm that  you receive the Result of Void.

8. Confirm the extension is no longer in the extension list.  If so, delete the VM snapshot created earlier.

Common vCenter extensions:

Extension Name	Service Description
com.vmware.vim.eam	vSphere ESX Agent Manager
com.vmware.vim.inventoryservice	vCenter Inventory Service
com.vmware.vim.ls	Licensing Services
com.vmware.vim.sms	VMware vCenter Storage Monitoring Service
com.vmware.vim.sps	VMware vSphere Profile-drive Storage Service
com.vmware.vim.stats.report	Performance charts built-in extension
com.vmware.vim.stats.vsm	Service Manager
cim-ui	vCenter Hardware Status
health-ui	vCenter Service Status
hostdiag	Internal extension to declare diagnostic events from VMware Host systems
VirtualCenter	VirtualCenter dynamic events and tasks
com.vmware.orchestrator	VMware vRealize Orchestrator plugin (formerly known as VMware vCenter Orchestrator plug-in)
com.vmware.rbd	Auto Deploy
com.vmware.syslog	VMware Syslog Collector Configuration
com.vmware.vcDr	VMware vCenter Site Recovery Manager Extension
com.vmware.vcHms	vSphere Replication Management (VRM)
com.vmware.vcIntegrity	VMware vSphere Update Manager Extension
com.vmware.vShieldManager	vShield Manager
vCloud Director-1	vCloud Director
com.vmware.vcops	vRealize Operations Manager (formerly known as vCenter Operations Manager)
com.vmware.vadm	VMware vRealize Infrastructure Navigator (formerly known as vCenterInfrastructure Navigator)
com.vmware.vdp	vSphere Data Protection 5.1
com.vmware.vdp2 com.vmware.vdp2.config	vSphere Data Protection 5.5/5.8
com.vmware.vsan.health	vSAN Health Check Plug-in
com.vmware.heartbeattasks	vCenter Server Heartbeat
com.vmware.hbwc	vCenter Server Heartbeat
com.vmware.heartbeat	vCenter Server Heartbeat
com.neverfail.heartbeat	vCenter Server Heartbeat