Based on the blog link below, the transformation to WGS84 seems like it would be embedded in the hosted feature service JSON somewhere. How can I find this? Doing some sniffing around, it seems like HFS published from ArcMap (10.4.1) has a spatial index set which references the transformation. However, I'm not seeing this in a HFS published from Pro (2.2)
When you need to branch out and overlay that data on basemaps that are not the same as your organization’s basemaps, you can pick the right transformation for your data before you publish. In the latest release of ArcGIS Online, you can do just that when publishing hosted feature layers. Any supported transformation you set in the map or data frame before publishing will carry through and be used when working with the hosted feature layer.