"What are you doing with your weird broken browser, you weirdo?"

(I am also a weirdo)

Fun fact, Slack also started blocking Fierfox 78 ESR last winter, which was the default browser in Debian stable until just a couple of weeks ago. They don't merely show an unsupported browser message, but actively prevent the app from loading. It still worked fine if you spoofed the user agent string to report a later Firefox version. It felt like unnecessary hostility to users for no actual security benefit.

Realistically, because if you are blocking Javascript in 2022, you probably aren't in their core demographic.

Slack, either the web version or the desktop version, will not work without Javascript. So its reasonable to expect that their customers will have javascript enabled (or more aptly, not-disabled) - allowing them to use more dynamic features to showcase their tool.

Probably easier to expect that the users browser hasn't loaded the Javascript and cause a page refresh to show them the site, than expect a non-Javascript user.