This was fixed as a result of a larger **raster options** issue. Raster proxies are now editable in ArcGIS Pro 2.2. The raster proxies folder does not apply to intermediate files from the Image Classification Wizard. That is still handled by the application's temporary directory. The raster proxies folder houses files that are created when georeferencing or calculating stats on a raster in a read-only workspace.
対処法
In order to place the raster proxy files on D drive, (or another location outside of C), open the environment variables and edit the TMP location to a folder of choice. ArcGIS Pro respects that setting. The one issue with this is that any other application using the TMP environment variable also uses the same temporary location for their files. To open the environment variable, go to Control Panel > System > Advanced system settings > Advanced tab > Environment Variables. For the TMP variable, change the value to the location where users would like ArcGIS Pro to add the temporary files.