According to the documentation it might only be available to Splash pages configured as click-through :
"/networks/{networkId}/clients/{clientId}/splashAuthorizationStatus": {
"get": {
"description": "Return the splash authorization for a client, for each SSID they've associated with through splash. Only enabled SSIDs with Click-through splash enabled will be included. Clients can be identified by a client key or either the MAC or IP depending on whether the network uses Track-by-IP.",
"operationId": "getNetworkClientSplashAuthorizationStatus",