how to block a specific URL without root? i want to block a specific URL (a .json file on a webserver)

i dont have root
it should run on ANY modern android device AND FireTV ;)

would be nice if we find a way ;)
The only option to block one specific url is using a HTTP proxy. Some HTTP proxies also support HTTPS decryption after installing the used root certificate into Android (AFAIR this is limited to Android up to Android 7.0). 
Also this only works for apps that use only Android system method for checking the server certificate (no certificate pinning in the app implemented).

There are some on-device proxy apps available, however I don't have much experience with those and don't know if you can configure one to block a specific URL. However it is possible. 

What definitely works when using Wifi is to use a HTTP proxy on your PC (Fiddler, Burp, Zed, Charles, ...) and redirect the traffic from your Android device to your PC and the proxy running on it. The proxy on the PC can be used to inspect, modify or block certain requests.