The URL for Quarry has been changed. Previously, it was https://quarry.wmflabs.org/. Now, it is https://quarry.wmcloud.org/. The old URL now redirects to the new one.
So, why has the URL been changed?
Topic on Talk:Quarry
We consider wmflabs.org hostnames to be "legacy" these days. Wmcloud.org hostnames are the common replacement. The Cloud VPS instances running Quarry were recently replaced with newer versions and updating the hostname for the service was a convenient side effect. As you noticed, the legacy hostname and URLs still work, but they will be redirected to the new hostname. There are more updates for Quarry planned for the coming months, see https://techblog.wikimedia.org/2021/09/02/digging-deeper-into-quarry/.
Maybe to reflect the fact that Quarry is "Hosted on Wikimedia Cloud VPS"?