Many things have changed in SharePoint-architecture from 2010 to 2013. In 2010 I would create it in ISAPI-Folder.
How do I create a custom web service in 2013? Hosting it in IIS? What App-Pool should I use? What about Authentification?
You can still create Web Services and put them in the ISAPI folder in SP2013. There may be a new best practice which I am not aware of. But it is technically possible.
Have you tried it and experienced problems? If yes, what problems?