skl2onnx currently can convert the following list of models for skl2onnx :skl2onnxversion:v. They were tested using onnxruntime :skl2onnxversion:rt. All the following classes overloads the following methods such as OnnxSklearnPipeline does. They wrap existing scikit-learn classes by dynamically creating a new one which inherits from OnnxOperatorMixin which implements to_onnx methods.
skl2onnx.algebra.sklearn_ops.OnnxSklearnPipeline
skl2onnx.algebra.sklearn_ops.OnnxSklearnColumnTransformer
skl2onnx.algebra.sklearn_ops.OnnxSklearnFeatureUnion
skl2onnx maps every ONNX operators into a class easy to insert into a graph. These operators get dynamically added and the list depends on the installed ONNX package. The documentation for these operators can be found on github: ONNX Operators.md and ONNX-ML Operators. Associated to onnxruntime, the mapping makes it easier to easily check the output of the ONNX operators on any data as shown in example l-onnx-operators.