Did some quick testing replacing Intersystems.Data.CacheClient.dll with Intersystems.Data.IRISClient.dll and ran against Cache 2018.1 and IRIS 2019.1. Seems to work in both cases. Is it a safe assumption that the IRIS dlls are backward compatible with Cache 2018.1?
It's not safe to assume compatibility between IRIS and Caché .NET gately DLLs. They may be coincidentally compatible at the moment, but as IRIS evolves, maintaining backwards compatibility will be a lower priority than enhanced features, performance and reliability. Is there a particular reason you tried this, or just curiosity?