Who runs it
Slack. It names itself with the token Slackbot. Slack runs three robots, each with its own user agent: link expanding, an image proxy and a general-purpose Slackbot.
What it does
The link-expanding robot responds to links that Slack users post into their channels, and fetches part of a page with HTTP Range headers to read its meta tags. The image proxy fetches and caches images posted into channels.
Renders JavaScript
Not yet observed. Slack doesn't say whether it runs scripts, and this site hasn't recorded a verified visit yet.
How to verify it
Slack documents no way to verify its robots. There is no IP list, reverse DNS check or signing method.
robots.txt
Slack says it does not currently honor robots.txt files. It says it is not a crawler, because it does not follow links and acts for a person. Site owners can ask Slack to block embeds.
Questions
Can robots.txt stop Slack expanding my links?
No. Slack says it does not currently honor robots.txt files, and site owners can ask Slack to block embeds.
api.slack.com/robots