Yay, json/index.json works similar to index.html — gets served at https://github-cben-sandbox.anat-beni.net/json/ (aka https://cben.github.io/sandbox/json/) — but with content-type: application/json; charset=utf-8 !
It's also 301 redirected from https://github-cben-sandbox.anat-beni.net/json .
This file is deliberately not named README.md because that got served instead of index.json.
N.B. You may find that having a file with the same name as the directory stops the redirect. For example if you have a api.json and a sub directoy called api containing a index.json and in the same root directory.
+-- api.json
+-- api
| +-- index.json
If this is the case simply delete api.json and /api/ & /api/index.json should be routable.