I don't really understand the outrage. The status quo is that companies didnt support it for years. So 99.9% won't notice any change.
But a mail CLIENT is a Web App not a static documents site. If Wikipedia would require JS I would kind of understand it from a technical point.
But big corporation tries to reduce cost by shutting down scarcely used old service happens monthly.
Sorry but there are so much more things you have a way bigger impact on climate than website requires a little bit JavaScript.