You should probably look at the test before making firm opinions on the matter. Do you even know what is failing?

Master is not production. The last release had all tests passing. https://travis-ci.org/developit/preact/builds/180853866

Sure it can. They can just say something like "IE9+ is supported, but features x and y won't work".

There are plenty of libraries that officially support IE, even when there are problems in a few small cases. They usually just make sure these issues are publicized so people won't be surprised them.

There are tests to validate IE support. The intent is clearly there. Circumstantially this design decision is not met, but it is clearly a bug not some fundamental lack of support.

Please consider how your feedback might affect the people giving their effort to offer an option to the community.