A WMTS with secured tiles but without a secured service does not display in ArcGIS Online.
Last Published: October 1, 2015ArcGIS Online
Bug ID Number
September 22, 2015
August 21, 2020
Operating System Version
7.0 64 Bit
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
Secured OGC layers are not supported in ArcGIS Online at this time: https://doc.arcgis.com/en/arcgis-online/reference/ogc.htm