Hi, it depends exactly what you mean.
For GeoJSON, TerriaJS will reproject vector data in almost any projection to WGS84 by using proj4.
There are hooks in GeoJsonDataSource to do the same in pure Cesium if necesary. For Esri FeatureServers, TerriaJS will turn any Esri "wkid" to "urn:ogc:def:crs:EPSG::<wkid>" and then try to reproject that using proj4.
For raster data, ArcGIS MapServers can usually do the reprojection themselves via the "export" service. However, if a pre-cached tile layer is using an unsupported projection, TerriaJS and Cesium can't use it. In theory they could still use the (slower) export service instead, if the server supports it (but it may not). You can force it to try using export instead of the pre-cached tiles by passing false for "usePreCachedTilesIfAvailable" to the ArcGisMapServerImageryProvider constructor (TerriaJS will do this automatically).