You can get around this by being selective with the apps on the home row. I have the Home Sharing, Settings, Search, Photos, and Podcasts apps on the home row. No ads.
Even then I don't quite consider it quite ad-free. Content I've purchased in iTunes Movies or iTunes TV Shows will always show the store on launch or will switch to the store if you pause content for some amount of time.
have you tried closing the tab and restarting the browser? failing that, there might be some mechanism where slack saves your timezone in its user profile (or localstorage) and uses that even if the timezone reported by the browser changes
(edit) Sorry, for previous users the preview site might be better: https://svelte-5-preview.vercel.app/docs/introduction