Unable to route 100% traffic to non-master branch in split testing

You cannot change the redirects after a site deploy; you’d deploy the different redirects while you deploy.

You could certainly change your production branch to a maintenance branch, and then create a 100% split test to it. But split testing is as Hrishikesh suggested a bit of an odd way to handle this, since it is a service in beta and has many caveats including around changing production branch on your site, so just publishing a different deploy is probably the most straightforward way to do this: