I will probably keep tracking the WHATWG living spec and never implement any other spec (old HTML versions, XHTML), so if you have a keen interest in anything other than contemporary HTML, SuperHTML might not be the right tool for the job.
That said, I'll add a warning if you put something else in the doctype so that users can know what to expect.
Now, separately from that, it might be nice if it told you that it doesn't support DOCTYPE == XHTML and will treat the document as HTML anyway, but that should only appear as an error/warning on the DOCTYPE line, it shouldn't confusingly mention XHTML wherever you might have meant some XHTML-ism that is invalid HTML.
Also, XHTML is not "an older version of HTML", it is a completely different standard that is (or was) sometimes used instead of HTML.