It seems there's no native WMTS support in Leaflet and there's also no maintained plugin available on npm. As such, this won't be implemented anytime soon as implementing WMTS natively for Leaflet for a nice-to-have feature is out of scope.
XYZ would be possible but it seems no service exposes XYZ previews at the moment.
The map with the extent could also show the XYZ or WMTS if the links are available in the metadata.
The issue could be that some WMTS services have timestamps and we could only show one timestamp easily, which might be misleading.