That doesn't seem to have anything to do with Cloudflare. Cloudflare isn't handling traffic for those sites. They both appear to be handled by Fastly. Also we are not down.
(Title was "Is Cloudflare down?" which is why I replied).
Also, if you hard reload Slack (ctrl-shift-r) the icons on the far left don't load.
Yesterday there was problem resolving cloudflare hosts when you used google DNS. Related?
see: https://twitter.com/vavrusam/status/1013120757809860609