Still the fetch method is there to worth with in modren browsers, so I though it deserves a quick post so lets do this.
If all I want to do with fetch is to preform a get request then making such a simple get request is real simple. In that case fetch just needs to be given one argument which is the URL of the resource that you want to fetch. This is a whole world more simple compared to using XMLHttpRequest.
If you are concerned about support for older browsers, then a polyfill for fetch will have to be used, or just do things the old fashion way with XMLHttpRequst as any polyfill will likely use just that. A popular polyfill for fetch can be found on github here at github.
Another option would be to just not use fetch at all and chose or make an http client, which is what I would often do rather than using fetch when it comes to working on an actual project where I will need to script http requests.
In addition I would not say that fetch is a replacement for various user space http clients that wrap XMlhttprequest rather then that of fetch. If a client is designed right there is a potential to get a best of both world situation between browser support and supporting later features like promises.