[terrywang]

Not on by default for 1 to 1 chat but it (Secret Chat) is optional, drawback is that the end-to-end encrypted conversation stays only on the devices initially it was initiated, because the encryption keys stay only on those 2 devices.

For Telegram Group Chat - it is not end-to-end encrypted, Telegram uses distributed infrastructure approach to protected data not covered by End-to-End encryption. I wouldn't consider WhatsApp's Group Chat end-to-end encrypted.