IE is, after all, a discontinued browser. While I'd expect most public-facing websites to provide support for it given the longish tail of legacy users, it's reasonable that some applications may no longer require that support, and it's good that libraries exist to fill this use case.

Makers of new libraries are free to chose how far they go with testing and support in different browsers. Of course you might be forced, by you client base (or intended client base) to support other browsers and if so you have three choices: don't use libraries that don't support the browsers you need to work with, test yourself and maintain your own branch if changes are needed for those browsers, or try nag people to share your pain and support what you need them to support.

Option three is likely to fall on deaf ears as the decision of how much time is worth spending on compatibility testing has already been made, so unless you can give a really compelling reason the won't want to. Option two is only viable if you (or your team more generally) have both relevant experience and time, which is often not the case. So this library may not be for you...

Not supporting at least IE11 is rare currently in my experience but I suspect it is a condition that will steadily grow more widespread. You may find that everything in the library works well enough in IE11 and the developers try to not intentionally do something that would change this, but don't feel they have the time to test (and fix as needed) sufficiently be able to make such guarantees.

Of course existing libraries that support IE11 are generally expected to keep doing so at very least least up to the next major release.

They state that older browsers and versions might be supported. I've tried this in IE 11 and it works like a charm, except for a couple of minor features. The diagram is the browsers that are "fully" supported.

Should they always have to?