yea, researching target audience is a good idea. But it might also be you need features not supported by IE. So then a decision needs to be made. Always a tricky balancing act. Experience at weighing the pros and cons is valuable here
you can check documentation (MDN, caniuse) for that. You don’t have to remember
Generally, there will be a deprecated warning in the console or in your text-editor or IDE. Then it often takes months if not years before the feature is actually removed.
Depending on how you build/setup your website, users might email you that some feature stopped working. Or maybe you have a monitoring tool and might notice something in unusual in this monitoring tool.