I assume you're not referring to things like json.Marshal(), where the function argument is legitimately of type interface{} since it really accepts any value.

Hence I'm going to assume that you're referring to situations where a value can have one of multiple types (e.g. a float value that sometimes gets provided as a string). It's true that people sometimes unmarshal into interface{} and then type-assert in those scenarios, but that use of interface{} is a code smell that should be flagged in review. The proper way is to unmarshal into a custom type with a json.Unmarshaler implementation that accepts all possible input formats (and vice versa for json.Marshaler if encoding is required). This is a real-world example from one of my codebases: https://github.com/sapcc/limes/blob/d833228e976ac1ddc4b95779...