Turns out it’s not only Windows 8 that has its telnet client disabled, Windows 10 is in the same boat. I’ve been using Windows 10 for quite a while now and just discovered this issue. Anyway, the way to enable is as follows:

For security reasons, apparently. I can see that making sense with the telnet server but the client? It’s an invaluable network debugging tool, after all, especially in heterogeneous networks.