Does anyone know the new Citrix Success Center URL (Currently in documentation https://success.citrix.com/ , even https://success.cloud.com/ is not working anymore) ?

Yep all that stuff is gone, and there are a lot of broken links throughout the Citrix site. The Success Centre was mostly just a bunch of links to Citrix doco anyway... You might find some of the old content on community.citrix.com https://cloudmicrositedemo.azurewebsites.net/ is still up and running though if that's of any use ;-)