I was wondering If I can still use the dafault Bing Map imagery provider in Cesiumjs sandcastle in the updated releases of Cesium js each month . and also how would be able to view other imagery providers in the Cesium js sandcastle too.
Yes you can! Bing Maps is the default imagery served through Cesium ion, so you just need to create an account and get a token, see: https://cesium.com/docs/tutorials/getting-started/.
In addition to imagery from Cesium ion, you can see all the built-in imagery providers in CesiumJS here: https://cesium.com/docs/cesiumjs-ref-doc/IonImageryProvider.html?classFilter=Imagery.