You mentioned "plain text" specifically - where else would they be holding those plain texts?

So far, there is no evidence to suggest your messages are stored in plain text. And in 2015, Telegram was using MTProto 1.0 for their cloud chat encryption and Secret Chats E2EE. It's been about 5-6 years since they've upgraded to MTProto 2.0 which has been proven to be a sound encryption protocol.

It was Moxie Marlinspike that also made the claim messages are stored in plain text on Telegram's server with no evidence. And so far, the only thing we have are hypotheticals and nothing of substance to support that claim.

The audit done in 2020 goes over how Telegram encrypts their cloud chats and those encryption keys are not stored on the same servers. While E2EE is preferable, the reason why Telegram works the way it does is because how messages are handled by default.

Hopefully soon they will roll out Secret Group chats. But I do like we all have the option to use Telegram however we want.

Where can I get info on Telegram storing messages in plain text on their servers? I have asked and searched and all I have seen are hypotheticals but nothing concrete.

I've read through the audit they had in 2020 where cloud chats are encrypted using the same MT Proto 2.0 which they also use for the secret chats (E2EE).

The same way that evidence is available, I would also like to see the evidence of cloud chats stored in plain text and not encrypted.