Thanks for your reply!
I did this
baseLayer:
Cesium.ImageryLayer.fromProviderAsync(Promise.resolve(
new Cesium.UrlTemplateImageryProvider({
url: "https://a.tile.openstreetmap.org/{z}/{x}/{y}.png",
}))),
and it seems to work. However, if I comment out the part to remove the icon, it’s still there.
viewer.cesiumWidget.creditContainer?.parentNode?.removeChild(this.viewer.cesiumWidget.creditContainer
Does this mean there’s still a link to Cesium ION?