I just verified in XrmToolBox - Metadata browser plugin, there are more than 400+ System entities (non-custom) available per
IsCustomEntity filter. You can connect to the environment, pull the metadata, move the columns you needed, export to Excel for analysis.
Along with my original custom entities, some LinkedIn integration entities &
msdyn prefix entities are also marked as non-system (custom) entities. So you have to use James mentioned
IsCustomEntity property to filter out the system entities.
Query to exclude system entities metadata while fetching:
api/data/v9.0/EntityDefinitions?$filter=IsCustomEntity eq true