I know it's already posible searching for "@username", but it would be nice to have a quick button to acces that mentions list, and maybe with a counter so we know we've been mentioned while we've been offline. That counter would clear only when you check on that list, so even if you open a group chat, which may have over 2k unread messages, you still know there's some part where they wanted you to read :)
+1
+1 mentions and replies
Something like this:

Clicking the mentions link would search for your @ handle (optimally only for the unread mentions).
up
A notification center...
@theus after reboot?...
six months later after reply/mention?...
@zaz600 I don't know if API allow this resource, but I think that Telegram knows if message was read or not. Something like this: the notification of reply/mention would stay in the center until you read. Or a history of these notifications.
History is also needed, not only unread
Also you can change the color of unread messages if they include mentions.
already implemented no?
Search for #m