Just to point out, you can (even today) in a plain discourse forum (without plugins) filter the replies to a post by clicking in the button at the bottom left corner of the post (The one that shows “1 Reply” or “X Replies”).
In your case, you could filter the replies to the first post in a topic (the post that created the topic). This assumes that:
- All the replies to the 1st post made using the topic reply button (instead of replying to another post, but referencing the 1st post).
- All replies to other posts are made clicking in the reply button of the respective posts.
The conditions above are what is expected anyway, but depends on the forum users replying to the correct post.