The reason for this change is that QueryProviderBase does not control what IQueryParser is used to parse the queries. Therefore, it has no access to the ExpressionTreeParser (which might event not be used at all).
To access the MethodCallExpressionNodeTypeRegistry previously available via the ExpressionTreeParser, you have the following options:
If you create the QueryParser in your code, keep track of it and use its NodeTypeRegistry property (or the registry you passed in when creating the QueryParser).
If you use the constructor of QueryableBase that automatically creates the QueryParser, you can cast the value of the QueryProviderBase.QueryParser property to the QueryParser class and use that class's NodeTypeRegistry property.