It has been brought to my attention that my vscode-map-preview extension now produces 401 responses for stamen map tiles, which has prompted me to put out a new release of this extension to better handle this situation.
This new release updates OpenLayers to 10.2.1 and now hides stamen tile layers from the base layer switcher if you don't have a Stadia Maps API key in your extension settings as this API key is now a requirement for viewing stamen tile layers.