Some Modern English spellings (15th century on) include:
threshould, thressald, threszshold, tresholde, thresholde, threshold, threskwolde, thresh-fod, thressholl, threshal, threshel, thrashel, drashel, thressholl, treshwart, threshwort, threshut and most interestingly freshwood (this last suggesting either a separate term that merged or an even more interesting possibility a folk-etymology pushing the word into a new direction).
I'm not even going to touch on the Middle English and Old English forms, beyond noting that they are even more various, and similarly its cognates in other languages are not always obvious.
It's a very old word that has had a variety of both spellings and pronunciations.
We can see in the above alone, some that seem to have no /ʃ/ at all (which in English we most often get from -sh) such as threskwolde, and some that clearly do, and likewise some that seem to have no /h/ and some that clearly do. Indeed, we've every possible permutation of whether it has /ʃ/ and /h/, /ʃ/ and no /h/, /h/ and no /ʃ/, or neither.
And it remains that /ˈθrɛʃəʊld/ (no h) is listed along with /ˈθrɛʃhəʊld/ as the pronunciation in the OED, with Mirriam Websters giving both too in their way of offering pronuncation guides ("\ˈthresh-ˌhōld, ˈthre-ˌshōld\") and so on.
And there we have it, the word is still found in two pronunciations, one of which matches the single-H spelling perfectly and one that doesn't, and both those pronunciations and the sole spelling that remains in contemporary Modern English come from a much greater variety earlier in the Modern period, which stems from a similar variety prior to that.
