There is an enhancement request that has been logged for this ability with the ArcGIS Enterprise core components (ArcGIS Server, ArcGIS Data Store, Portal for ArcGIS), but the functionality from a web-tier perspective is already available depending on the configuration of the web server running the ArcGIS Web Adaptor or whatever load balancer/reverse proxy is fronting the client-side communication to the internal components.
My configured HTTPS binding for port 443 (using IIS 10.x) shows in my browser's network traffic capture as using HTTP/2 for communication with ArcGIS Enterprise resources when accessed via the web adaptor.
-- Chris Pawlyszyn