Slack threads don’t (or at least, didn’t; it’s been a couple years) show in the main message feed, they show when you expand the thread into the sidebar. In the feed, it shows the original message with an “n replies” text below it.
In Mattermost, new replies show as normal messages in your feed with a bit of context for what they’re replying to, and clicking on the context expands the full thread in a sidebar.
I find the Mattermost implementation more ergonomic because you can read replies without expanding the whole thread every time, and stale threads will get bumped by new messages.