ArcGIS Enterprise to ArcGIS Enterprise collaboration
The minimum version for ArcGIS Enterprise to participate in a collaboration with ArcGIS Enterprise is 10.5. Keep in mind that any participants using earlier versions will only have capabilities available at their version. This may affect other collaboration participants, even if they are on a later version of ArcGIS Enterprise. For example, a collaboration between ArcGIS Enterprise 10.5 and ArcGIS Enterprise10.7 will only be able to share feature layers by reference, as the ability to share feature layers as copies was not a feature at 10.5.
ArcGIS Enterprise to ArcGIS Online collaboration
The minimum version for ArcGIS Enterprise to participate in a collaboration with ArcGIS Online is 10.5.1. Collaborations between ArcGIS Enterprise and ArcGIS Online will be limited to the features that are available at the Enterprise deployment's current version.