There is no standard naming of keys in JSON. According to the Objects section of the spec:
The JSON syntax does not impose any restrictions on the strings used as names,...
Which means camelCase or snake_case should work fine.
Imposing a JSON naming convention is very confusing. However, this can easily be figured out if you break it down into components.
Programming language for generating JSON
JSON itself has no standard naming of keys
Programming language for parsing JSON
snake_case will still make sense for those with Java entries because the existing JSON libraries for Java are using only methods to access the keys instead of using the standard dot.syntax. This means that it wouldn't hurt that much for Java to access the snake_cased keys in comparison to the other programming language which can do the dot.syntax.
Example for Java's org.json
package
JsonObject.getString("snake_cased_key")
Example for Java's com.google.gson
package
JsonElement.getAsString("snake_cased_key")
Choosing the right JSON naming convention for your JSON implementation depends on your technology stack. There are cases where one can use snake_case, camelCase, or any other naming convention.
Another thing to consider is the weight to be put on the JSON-generator vs the JSON-parser and/or the front-end JavaScript. In general, more weight should be put on the JSON-generator side rather than the JSON-parser side. This is because business logic usually resides on the JSON-generator side.
Also, if the JSON-parser side is unknown then you can declare what ever can work for you.