I understand why people don't like this change (limiting options, etc), but I can see slack's POV from a business perspective. The mobile web usage might be so low that maintaining it costs more than forcing people to switch to the app. And mobile web might be a bad experience anyway (because slack is focused on standalone desktop app, mobile app, desktop browser instead), so it turns new users off with the crappy experience. But I don't have the numbers or any other insight, so it could be more simple: "we want more app usage". And even if you decide to just leave it alone, is anything truly maintenance free? (possibly)
This won't be the last thing to turn off mobile web to force-guide users to the app. And luckily for actual mobile web users, there seem to be alternatives.
This won't be the last thing to turn off mobile web to force-guide users to the app. And luckily for actual mobile web users, there seem to be alternatives.