Per the announcement from Stamen, the old Fastly CDN URLs have been shut off for a bit. We have a migration guide that covers the necessary steps to get this back up and running.

TL;DR steps:

1. Change the URL pattern to something like https://tiles.stadiamaps.com/tiles/stamen_terrain_background/{z}/{x}/{y}{r}.png (note that these tiles have ben updated and support retina/hidpi now!)
2. Update attribution / license information as noted (Stamen, Stadia Maps, and OpenMapTiles need attribution + links now).

I actually came across this repo a while ago and meant to open a PR last month, but somehow missed it so dropping an issue now to track it since it's the root cause of facebook/Rapid#1241.