It was my reading of the 10.5.1 deprecation notice.
10.5.1
Support for cluster functionality
Esri is announcing the deprecation of the cluster functionality within the ArcGIS Server component
of ArcGIS Enterprise. Instead we recommend creating separate ArcGIS Server sites where multiple
clusters would have been used previously. While we are not announcing a specific future release
where cluster functionality will be removed, the use of multiple clusters for new deployment is
strongly discouraged and existing deployments are advised to migrate away from use of clusters as
part of ongoing maintenance and upgrades.
Note: this does not affect the ability to create multi-machine sites.
ArcGIS Server sites with multiple machines continue to be fully supported.
vs. 10.6 deprecation notice
Support for Multiple Clusters with ArcGIS Server
ArcGIS Enterprise 10.6.x will be the last major release series to support multiple clusters with ArcGIS
Server. This was previously deprecated at ArcGIS Enterprise 10.5.1 and we are n
ow announcing the
official removal of that feature from the product in the next major release. Users with multiple
clusters are strongly encouraged to migrate to a different deployment pattern to avoid disruption at
later releases.
