Use specialized browsers like Chromium-Legacy or Floorp (a Firefox fork), which are actively maintained to provide modern web support for older macOS versions.
If you must use Slack on a High Sierra machine, here is how you can still access it using legacy software or alternative methods. 1. Download Legacy Slack Versions slack high sierra download
If you have a slightly newer version of Google Chrome installed on your High Sierra machine, you can "install" Slack as a PWA. Open Chrome and navigate to your Slack workspace. Go to the in the top right corner. Use specialized browsers like Chromium-Legacy or Floorp (a