The following code pulls a full channel history from the Slack API, exports the message and user data to .json files, then parses an HTML mockup of the history in a similar style to Slack.
Am installation of Node.js is required in order to run this Slack scraper. Download the packaged installer directly from nodejs.org or if you're using a Mac, you can install it with Homebrew.
Open a terminal window and run:
$ brew install node
To use, set the token variable to your legacy user token;
(Issue a legacy token from the Slack API Help Center)
then set the channel variable to the internal channel id of the channel you want to scrape.
(Find a channel's id with the test tool in the Slack API documentation)
Finally, in the console, cd into the directory and run:
$ node app.js
then open data/history.html in a browser.