CPU features are exposed to VMs at power on, i.e. VMs which have been powered on on the new "Cascade Lake" host run with features, which are not available on the "Broadwell" hosts.
In order to enable EVC for the cluster, you need to shutdown all VMs with the newer feature set, i.e. the ones which cannot be vMotion'd from the new host to the old ones. As an alternative to shutting down all these VMs at the same time, you could shut them down - one after the other - migrate them to one of the "Broadwell" hosts, and power them on there again. After that you should be able to enable EVC with "Broadwell".
Please remember to disable "Initial Placement" (i.e. set "DRS" to Manual in case it is enabled), to ensure that the VMs are powered back on on a "Broadwell" host.
André