Removing support for a browser should be very carefully considered and with lots and lots of notice to users. The next shiny thing that the old browser cannot accommodate may not be something that most users care about, though developers always want to be cutting-edge.
I am concerned about the users who are required to use a particular browser more than those who have free choice. I would like to hear more about the reasons—corporate culture can be changed, technical ability of devices perhaps not so easily, does governmental regulation or censorship come into play at all?
From a sustainability perspective, smaller files mean quicker speeds and less energy—until the next iteration of features clogs everything again.