Is the arcpy function in python 3.x to create an arcgis server connection file (*.ags) exposed like CreateGISServerConnectionFile
I think the short answer is still, you don't, unfortunately. The Create GIS Server Connection File tool isn't/wasn't a core geoprocessing tool but an ArcPy Mapping function, and the ArcPy Mapping module was completely overhauled between ArcMap and Pro. I haven't double checked in Pro 2.7, but last time I looked into this issue there was no good solution.
UPDATE: See https://community.esri.com/t5/arcgis-pro-questions/how-to-create-an-arcgis-server-connection-file-wi...for an option on using the undocumented arcgisscripting._createGISServerConnectionFile.