Canceled Secret Chats remain in the chat list, you can view messages from them but can't send new ones: a Delete & Exit button appears instead of the input field.
Secret chats will appear as canceled to the other user in these cases:
But there are cases when the secret chat will appear normally, with an input field for new messages, even if nobody can read them on the other end:
This happens because Telegram has no ways of knowing that your chat partner uninstalled the app or deleted its data. You will still be able to send new messages into such chats. Nobody else will be able to receive and decrypt them.
If C or D is true, your chat partner can go to Settings > Devices and terminate their old session to cancel all Secret Chats assigned to that session.