How one can know that the other participant has left the conversation?
They stop posting.
Is the name of the other participant who has left the conversation still there in the conversation participant?
Yes. And that can cause confusion.
No other ways?
Not unless they post that they are leaving the thread.
Usually leaving a conversation is only needed when there are 2 or more participants.
I am talking/writing about the case when two participants are there
Then leaving the conversation is quite pointless, you can simply ignore further messages, with the same result.
If you leave it with "accept further messages" leaving the conversation does nothing (apart from cleaning up) because you get in again if there are new messages.
Removes it from your conversation list
That's what I mean with "cleaning up".
Separate names with a comma.