I think this would be harder than you might think. I assume WaniKani is using modern features for both site layout and functionality. As a simple example, I assume WaniKani will be using some form of asynchronous request during reviews to check if the answers are correct. Historically, if you wanted to make such a request, you’d have to treat IE as a separate case just to fetch the content.
And then there’s the fact that due to IE officially not being supported anymore, a lot of libraries have already dropped support for IE, which would mean they’d have to implement the functionality themselves or find other libraries to replace them, which would require a refactor of the entire site.
Then there’s newer markup to consider to make sure your elements end up on the screen in some logical positioning. IE doesn’t support modern layouts (or CSS3 at all), so they’d have to rewrite most of it solely for IE.
And since IE is no longer maintained, it also doesn’t support a lot of modern standards. For example, take secure connections, IE doesn’t support TLS 1.3 (the newest secure connection protocol), and most versions of IE require you to manually enable TLS 1.2 (the version before that), everything older than that is already deprecated, so I assume the site has these disabled for security reasons. Thus most versions of IE wouldn’t be able to actually connect to the website at all if they want to ensure safe connections.
So all in all, there would be a lot of hurdles just to get WaniKani to be functional on IE. And since it’s quite outdated, I’d say it’s probably more trouble to support IE than it would be for most people to use a more modern browser. Unless you’re specifically targeting a demographic which uses IE, it’s not worth supporting it in this day and age.