You can retrieve all the activities information and journey metadata information using the REST API. See example here
However, I don't think there is any API to retrieve the audience list for a particular activity in a journey.
To get the audience list who have exited after a specific decision split, follow the below steps
- Create a boolean field ExistedAfterDecisionSplit in your entry source DE
- In Contact Builder->Data Designer, create a new attribute group and link your entry source DE to the Contact key using a 1:1 relationship.
- Create an update activity after that the specific decision split, choose the entry source DE from the wizard and updated the boolean field "ExistedAfterDecisionSplit" to true. This field will be true for all contacts who have exited after that specific decision split.
- Now you can create a filter DE on your entry source DE with filter criteria ExistedAfterDecisionSplit = true and target those contacts.