See also discussion at T240960
Right now there is some differences between prod and beta CSP. Since beta should be testing for prod, we should minimize differences.
There is a question of: Do we put sources in CSP for prod wikis (So you can say load your enwiki gadget from prod). I'm not sure, but i think it makes the most sense for beta to be as much like prod as possible, in which case that means only allowing domains that are in the beta cluster.