So today's release of ArcHub does not automatically forward all URLs to the new URL if there is an underscore in the feature service name.
We use links in our public web pages to the downloads in ArcHub
It took us a while to figure this out today, but all counties with underscores in them (e.g. San_Diego) forwarded to 404s
e.g
old map URL

If you replace the underscore with dash it works fine:
https://cdtfa.hub.arcgis.com/datasets/CDTFA::san-diego-2021-roll-year/about?layer=0.
Maybe ESRI will fix this but until they do this might help others.
Has anyone else encountered this?