The WoD leak turned out to be true.
The week before Shadowlands was announced, Blizzcon merch featuring Bolvar and his hammer was seen, so it was a dead giveaway about the nature of the expansion cinematic.
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Unless a company "leaks" info a few days before an announcement, generally no. Companies dislike leaks for a reason: things can change in development, and a leak can give customers skewed expectations. The eyes of a customer they have been promised something... only for that thing to maybe axed or changed before final release, so they feel lied too. Look at the backlash FFXIV faced when some stuff that was leaked was changed or axed during development (namely Blue Mage becoming a limited job). Blufevr said that Blue Mage was going to be a regular job, but when it actually came out it was a limited job and everyone was furious and the backlash against Square was pretty big. After that, Square apparently did a purge to try to stomp out Blufevr (who unintentionally narrowed down which department in Square he was in through the info he gave in one of his leaks that only his department knew about).
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Also, if something is leaked too early, it hurts the marketing cycle. The marketing cycle is predicated on releasing new bits of information over time. But, look at Genshin Impact. The second region was leaked 8 months ago, and it's still not out yet. That leak has killed Genshin's marketing cycle... because the fans know what's coming and there is hardly any new news for the devs to release. "Hey guys, we have new news to share with you! A second region is coming!" "yeah we knew 8 months ago, do you have any actual new news for us players?" "uh... no..."