Like many people in a relationship, I have a persistent backchannel with my wife. I have never used WhatsApp, and so we ended up using Telegram. After reading this article from the EFF, an organisation I donate to on a monthly basis, we’ve switched to Signal.
My wife’s family moved to Signal after one of the privacy debacles around data sharing between WhatsApp and Facebook. Many people I know have switched from Telegram to Matrix for group chat.
So the only people left on Telegram that I contact regularly are my parents, my sister, and a few random people I probably haven’t messaged for a while…
If you do not have [Telegram’s] secret chat turned on, your chat communications can be exposed or seen just like channels and groups. If you do turn on secret chat, then Telegram cannot see the contents of your communication, but they still have access to metadata about the communications, including who you talked to and when you talked to them. It may be possible to draw very specific conclusions about what you are doing based only on the metadata about your conversation.