All mentions of subversion seem to be gone from XCode 9, but it still works, and it's actually terribly simple:
(tested with XCode 9.4.1)
- Go to Source Control->Clone...

- You've probably seen this screen and stopped, because it says GitHub prominently. BUT, notice the "Enter repository URL" text bar at the top! You can just write your subversion URL there and hit Clone!

- Like so. As you hit the Clone button, you'll be asked to enter your svn credentials (if needed), and the directory where the repo will be cloned.

- After that, just open your project or workspace from the cloned dir, and you're all set. Even the new Source Control Navigator (command+2), should work right away.

I haven't figured out how to add things to a subversion repo, but for existing repos, this works. I've not tried this on XCode 10 beta at this time, either.