There is some level of backward-compatibility as I can connect to both Cache & IRIS using the single dotnet framework IRIS driver.

What is not clear is what is supported in different versions of the driver.

I can connect to the Cache version given in my post using the .Net Framework IRIS driver (InterSystems.Data.IRISClient.dll) version 2018.2.0.415, which is what we used pre-netCore upgrade.