As announced on June 22, 2020, the Get all valid channel access tokens v2.1 API endpoint is now deprecated and will be permanently removed on July 29, 2020.
If you're using this endpoint, change your bot to use the newly added Get all valid channel access token key IDs v2.1 endpoint by the specified EOL date.
Learn more about Issue channel access token v2.1 in the Messaging API documentation.