That’s what we’re arguing. Slack should not be able to restrict me from modifying the experience on my hardware.

Those terms bind Slack users. This problem goes away if the maintainer doesn't use Slack and has never used Slack. It wouldn't be efficient, but it'd be possible.

If you are using IE to browsing slack, and IE couldn't correctly render it. It is illegal?

JavaScript runs on the client genius, not the server. You're not changing their product, your modifying how it is run on your personal machine.