Google has deprecated the legacy FCM API in June 2024: https://help.twilio.com/articles/20768292997147-Updating-Twilio-Push-for-FCM-HTTP-v1-API
Detected the use of a legacy FCM server key credential. This credential has stopped working after Google has deprecated the legacy FCM API in June 2024. Your action is required in order for FCM push notifications to continue to work as the legacy FCM API is deprecated.
Use a new FCMv1 credential to keep sending push notifications. See instructions linked in the description above.