In geowebcache/core/src/main/resources/geowebcache.xml
(Current main at the time of writting)
this still points to the previous domain which should be deprecated from the whole repo to prevent confusions, and as it also seems the domain was taken by an external actor.
Which woould be the correct domain to point to obtain the geowebcache.xsd files now? they dont seem to be available either in a direct replacement https://geowebcache.osgeo.org/schema/1.28.0/geowebcache.xsd