Benefit of the dobut Reasoning: Engineering determined that the cost of supporting a mobile site outweighs the benefit given they have a great mobile app. Then, someone decided to disable mobile access, because it better communicates that they don't support mobile web; cusutomers may still use the broken, unsupported experience, and then they'll complain about it to their support. Thus, raising the cost of maintenance while lowering customer experience.

Keeping mobile web but adding a banner that the experience isn't supported: Doesn't Work. Most people are total idiots when it comes to tech; this is 250% more true in the B2B space; this is a further 250% more true in a bottom-up corporate sales model like what Slack deploys. Unsurprisingly, but likely; many of their workspaces are shadow-IT operations created by some do-gooder who knows Excel and thus he's a "tech expert", on an isolated team within a large company who's IT department really wishes everyone would just use Teams and Sharepoint. This person is going to dodge the company's frontline IT support with any issues they run into. This person probably has a company-issued phone and thus doesn't want to, or can't, install the app.