Hi James,
The model version can be identified in the asset package with name "_Version" table.
Use arcgis pro to open the AssetPackage geodatbase where under the the database folder. Open the _version table that shows what's the model version of this asset package.
I've check this "Water Distribution Data Management For ArcGIS Enterprise v1.1" . as your target Enterprise 10.9 and ArcGIS Pro version 10.8.x that you are going to use the UN model 5.
In order to match to the ArcGIS Pro version, that you can execute the Upgrade Asset Package geoprocessing tool to upgrade UN AssetPackage's model. Utility Network Package Tools => Asset Package => Upgrade Asset Package.
Below link shows the difference of the model version
https://pro.arcgis.com/en/pro-app/latest/help/data/utility-network/utility-network-upgrade-history.h...
The statement of not able to use Water UN model using the ArcGIS Pro 2.8.x might mean the default Asset Package of Water Solution is model 4.x and require upgrade ? I did not have a chance to set up 10.9 environment and Pro 2.8.x for testing.
My testing step will be :
1. Download the solution
2. run/test the UN function direct using the Map and from WaterDistribution_UtilityNetwork.gdb (not require any configuration)
3. Test deploy the asset package : upgrade the asset package to match the Pro version
4. Deploy in the fgdb : Exam if any errors
5. Deploy the asset pacakge to Enterprise GDB against 10.9 (pro user can be license by any method. when running the UN deployment, requires to add the Portal or AGOL connection, the login user has been assigned UN Extension)
~Vicky Wang~