According to OS docs the API enpoint used by this package is deprecated and will be disabled soon:
https://documentation.onesignal.com/docs/keys-and-ids#organization-api-key:~:text=On%20the%2014th,Keys%20%26%20IDs%20page
Legacy user API keys will be deprecated on the 1st of March 2025, and legacy app API keys will be deprecated in Q1 2026.
And to update you must update endpoint:
Make sure your API requests are using https://api.onesignal.com and not the legacy https://onesignal.com/api/v1/ URL.
Current package uses https://onesignal.com/api/v1'