[Nextgrid]

They can't see the content of the Slack channel. But they can absolutely correlate the web request that loads the GIF with the other (illicitly-collected) data they have on you and have an even better signal on who you are, which companies/people you talk to, etc.

[paxys]

All they can see is that a user on slack.com is requesting the GIF. If you are using the Slack desktop app there's no other session to correlate it to.

[Nextgrid]

The IP address and user-agent is enough of a session over time (remember that it loads every time you open your client as long as the GIF is in your recent history, so you get multiple data points to refine your search). Cookies or browser-specific state is on its way out anyway as more and more browsers impose restrictions.