The error message, "WFS service doesn't support GeoJSON output format" is returned in ArcGIS Online Map Viewer. The same service in ArcGIS Online Map Viewer Classic renders without the error. The GetCapabilities URL indicates Web Feature Service (WFS) 2.0 and the GeoJSON output are supported.
This issue is not reproducible in ArcGIS Online. If the issue occurs, please contact Esri Support Services as additional information is required for further investigation.