Earlier quoted context omitted.
As another HN user pointed out Telegram does not store messages in plaintext: https://news.ycombinator.com/item?id=41348228
That user seems to be misinformed, and appears to be discussing client-server encryption, not end-to-end encryption. That's unsurprising, because, among the many decisions Durov has made that have baffled cryptographers, attempting to confuse users about the implications of E2E vs. client-server encryption is one of the most notorious.
Telegram does not store messages in plaintext. Period.
No matter how shrill the cries from Moxie Marlinespike and his adherents, E2EE is not the only form of encryption.
MTProto 2.0 is fully documented and everything the user linked described is true.