Title. I'm not sure how to find out about it so I'll ask in here.
Shalom!
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Sign up to join this communitySiddur Hameforash by Rabbi Yaakov Wingarten brings the following:
See חיי אדם כלל ה’ סעיף כ"ז, where he states that it is prohibited or "איסור חמור” in his words, to pronoune the Hay.
The חתם סופר in שו"ת חו"מ בהשמטות סי’ קצ"ב also states that the minhag is not to pronounce the Hay.