I want replies to a topics to be organised in tree structure like other forums scripts have. I’ve done some searching and can’t find any solution and I also came to realize that Discourse was never developed in that way. The replies are mostly few years old so not sure if it has changed?
I was wondering if tree structure for replies can be achieved or it’s just a total no? If yes how?
Until someone comes along and quotes an authority, I’ll give you the short answer, which is No 
When people have clicked the small reply-to-post button instead of the main reply-to-topic button it’s possible to collapse/hide the other replies, but that only goes half way to what you want.
You can write a theme component (or maybe a plugin! IDK) to stylize the replies to your preference. That being said, it is not directly supported and there is not setting to configure in discourse core that would enable that functionality.
It’s so confusing when you are coming from other forum scripts. I’m 2 days and so far not able to get my way around it.
If I’ve to write or know how to write plugin I would build my own forum software.
If you click the little 1 (or probably 2 now) on my first post above, you’ll see our conversation with the other replies hidden.
That’s even better! Total control on your own software, Just the way you like it
Discourse follows the flat model for discoussions:
If you are looking for a threaded/tree-like software there is
I had misunderstood it myself. I think doing this shows all n replies to the relevant post but only the direct replies. So after this second reply to your post the number beside it might be 2 and it’ll show my two replies (but not any future replies to my own posts).
What forum software are you using daily? Do/did you not use forums like phpBB, vBulletin, Vanilla, etc?
I don’t know your goal (create a new community?), but I’m sure you’d be able to get your way around it fairly quickly. “Flat” discussions are quite common on the Internet.