No, the pronunciation changed. The -z at the end of a Dutch surname is not said as "zoon" but simply as a single letter, which due to final devoicing in Dutch becomes an /s/ sound. So you would greet Mr. Jacob Cornelisz as "Meneer Kor-nay-lis" (rhyming with "miss"), not "Kor-nay-lis-zoon".
The historical shift is exactly as you describe: names like "Corneliszoon" were shortened in writing to "Cornelisz", but the spoken language also evolved. By the time the spelling was standardized, the full "-zoon" pronunciation had already been dropped in everyday speech. Modern Dutch consistently applies the rule of final obstruent devoicing: a voiced consonant like 'z' becomes voiceless at the end of a word (so /z/ turns into /s/). Think of how "huis" (house) is pronounced with an /s/, not a /z/ , the same logic applies to the final 'z' in these surnames. Some families may have preserved a longer pronunciation in certain dialects or as a personal choice, but the standard, neutral pronunciation is the devoiced single letter.
For a learner, this means you can confidently pronounce any surname ending in -z (such as Pietersz, Jansz, or Dirksz) by saying the preceding part of the name normally and then adding a crisp /s/ sound. Avoid trying to insert a "zoon" syllable , it will sound unnatural to native speakers. To practice, listen to Dutch news or radio where surnames are often used; pay attention to how announcers say names like "Rembrandt van Rijn" or "Johannes Vermeer" , you won't hear a "zoon" extension. If you meet someone with such a name, you can also politely ask, but the safe bet is to use the single-letter pronunciation.
One more nuance: if the name appears in genealogical contexts or very formal settings (like a historical reenactment or a specific family tradition), the full "-zoon" might be used. But for everyday encounters, books, and modern media, stick with the /s/ sound. This matches the pattern of other Dutch abridged suffixes such as -sz (from -szoon) where the 's' is also pronounced as /s/. You can find audio examples in online pronunciation dictionaries like HowToPronounce or Forvo by searching for names like "Cornelisz" or "Pietersz". There's no need for a special course; just apply the devoicing rule and you'll sound natural.