If you index shingles (phrase chunks) instead, you lose out on sloppy phrases...you can only match exact phrases. I imagine you can perform a similar statistical attack too.

Hell, just getting the term dictionary would probably allow you to reverse engineer the tokens, since written language follows a very predictable power law.

Hashing also removes the ability to highlight search results, which significantly degrades search functionality for an end user.

Basically, yes, you can do search with encrypted tokens...but it will be a very poor search experience.

It's configurable for paid accounts, and can be set as low as one day. However, one of the best features of slack (and products like slack) is message history and search. Otherwise, IRC isn't all that different (WRT messaging).

First: Slackbot. This is a Slack-run bot that's in every channel; team owners can customize it to do various things, like scan messages for keywords and give out canned responses. Even if Slack adopted some variant of encrypted chat, each message would still need to be readable by Slackbot, so Slack would still have the means to collect every message.

Second: channel history. When I join a channel, I can see the messages in that channel from before I joined. This means that Slack (the server) must be able to give me those historical messages. In an encrypted group chat, the messages are encrypted only with the keys of the participants at that time, which means newcomers can't read them.

I'm sure there are other features in conflict with end-to-end encryption, too; these are just off the top of my head.

As for the second, the server could ask one of the clients to re-encrypt the channel history with the newcomer's key. It would only fail if nobody was online the moment you joined the channel (and you still could get it later).