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Why is threshold pronounced "thresh-hold"?

If people are pronouncing it that way, it might be one example of the spread of literal pronunciation in the last few decades. This is a trend where, contrary to traditional practice, people are pronouncing certain words as if every syllable needs its proper exposure.

One example is accent which until very recently would be pronounced acc'nt - that is, with the second syllable unstressed, a nothing-syllable, the way we (still) pronounce decent. But nowadays it's more commonly heard as a spondee, which is the technical name for a word with two equally stressed syllables. So it comes out almost as if it's two separate words, ax and ent.

More examples:

Philharmonic. Radio announcers traditionally suppressed the "h" in what is an unstressed syllable: philermonic. But now, as often as not, it's restored: phil-harmonic, as if it's two words.

Tortoise: traditionally tortus, increasingly tortoyse. Because, presumably, it accords with the way it's spelt. So, like accent, it's turning into a word with two equally stressed syllables, a spondee.

Maidstone. The traditional pronunciation, Maidst'n, still holds sway (I think), but increasingly Maid-stone is being heard.

So it might be that thresh-hold is a symptom of the same thing. The word's spelling is derived from two words joined together, but the second "h" is missing, probably because that reflected the standard pronunciation of former times: thresh'ld. But if you restore it in the spoken word, you're actually pronouncing a letter that isn't there.

German s is never pronounced as 'sh' it's mostly the 'z' sound in English, except for end of syllable, when it's pronounced 's'. Only ss or ß are always pronounced as 's', whereby ss shortens the vowel while ß does not.
Here the ethymology of threshold:
Old English þrescold, þærscwold, þerxold, etc., "door-sill, point of entering," a word of uncertain origin and probably much altered by folk-etymology.

The first element probably is related to Old English þrescan (see thresh), either in its current sense of "thresh" or with its original sense of "to tread, trample." The second element has been much transformed in all the Germanic languages, suggesting its literal sense was lost even in ancient times. In English it probably has been altered to conform to hold.

Liberman (Oxford University Press blog, Feb. 11, 2015) revives an old theory that the second element is the Proto-Germanic instrumental suffix *-thlo and the original sense of threshold was a threshing area adjacent to the living area of a house. Cognates of the compound include Old Norse þreskjoldr, Swedish tröskel, Old High German driscufli, German dialectal drischaufel. The figurative use was present in Old English.

This word is derived from old German. In the German alphabet the letter s is pronounced with a "sh" sound, and a double s is pronounced "es". The double s in German is ß and sounds like our English s. That having been said, the word gets pronounced thresh-hold and not thres-hold.