That looks identical to Slack's implementation to my eyes. If you don't mind sharing, what did you like better?
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.
It's worth noting that the design you prefer isn't being deprecated; Slack-style threads are being added as a user display option.