It seems like this happened quite some time ago and it's not because of anything NordVPN did. Companies generally don't make announcements when they don't support a product on another product, or stop supporting it. NordVPN isn't excluding Silverblue specifically, they're just not supporting rpm-ostree. Not their fault that Silverblue uses it. With the thousands (okay maybe hundreds but they keep increasing) of Linux distros no software provider can test with them all or guarantee they'll work even if they DID supposedly work the same way as one that they do support, and they obviously will only put resources into supporting those that are going to cover the most customers, i.e., the package managers that are most popular.
And yes, NordVPN like others appears to provide configuration files to load into the open clients.