You have an amazing product that's truly accepted as #1 chat application for offices around the world.
Now, what I dislike, and it started happening just recently, is you enforcing users to download application on mobile devices and cutting off your website for mobile.
Showing a message to download slack application for better experience is totally fine, but you shouldn't enforce it.
Have you considered that some of us don't have slack installed by choice? I don't want to have it easily available on my phone and catch myself checking slack after work hours the same way I check social networks. But I do want to be able to open it when I know something urgent is going on and requires immediate attention. I don't want to install application for that, and muting notifications isn't enough.
I think that's common enough usecase to keep slack website around. I don't expect perfect experience, and I know I have a choice to install the app, but please don't enforce it. I want to keep that choice around.