Private and inexistent datasets should return 404, not 401
Try 

https://datasets-server.huggingface.co/splits?dataset=severo/test_private_datasets
https://datasets-server.huggingface.co/splits?dataset=severo/inexistent

in a private window. It returns 401, not 404.
Private and inexistent datasets should return 404, not 401: Try 

https://datasets-server.huggingface.co/splits?dataset=severo/test_private_datasets
https://datasets-server.huggingface.co/splits?dataset=severo/inexistent

in a private window. It returns 401, not 404.