It is ridiculous that not only was this feature removed without warning, but not a single MS employee or support team member seems to know why or how, or will even fully acknowledge that this was taken out by Microsoft and isn't a new "feature" of "new" Teams. Not that it matters which Teams you're using anymore, as apparently the channel features from new Teams have been crowbarred into old teams with recent updates so now this option gone everywhere except the iOS app, which still shows the button but only the option to "Turn On Notifications":
Selecting the button does nothing. How is it possible that the team working on the mobile app clearly had no idea this feature was going away? It is like every different version of teams is being built by completely separate companies that refuse to communicate with each other.
This is unacceptable behavior for a flagship product of one of the biggest tech companies in existence; this is a critical feature as many others have said in this thread and we need some answers and a timeline on it's re-implementation ASAP Microsoft!