Reply in thread opens a draft: nothing is made, and nobody sees “View thread”, until the first reply is sent — and if that reply cannot be sent, the empty thread goes too. The person whose message started a thread, and everybody it named, follow the thread from its first reply, unless they left it.
Search finds “1-1”, “+1”, “#1”, “T-5” and “a-b” in messages, canvases and file names, and an apostrophe is any apostrophe wherever names and words are matched: “D’Souza” is found by “d'souza” in people, lists, canvases and files. The mention list matches where a word begins.
A channel you muted, or set to Nothing, no longer counts an @channel or @here; a mention by name still counts. An @channel counts in the sidebar as it arrives, and only in channels you are in. A message deleted while Activity is open leaves it at once. Put previews back is offered after a reload too.
Naming somebody who is not in the channel tells them nothing, as Slack has it: you are told, with Add them, Let them know or Do nothing. Pins say what a pinned file, sticker, poll or GIF is.