Hi Andy Egleton ,
To be honest, I haven't seen any functionality to test is something exists depending on the version. Attribute rules are executed on a database level. For instance, ArcGIS Pro 2.5 has support for it when you use a FGDB. However, when you point to an enterprise geodatabase and it is part of Enterprise 10.7, I don't think $originalFeature will be available. It was introduced at version 1.9 (January 2020), see: Release Notes | ArcGIS for Developers. And when you look at the Version Matrix | ArcGIS for Developers you can see that Enterprise 10.7 has Arcade version 1.5 and Arcade 1.9 was introduced in Enterprise 10.8.