I don't think most people would call that an official announcement. How is the public supposed to know if there as not been an announcement made on news feeds and social media? I would call this more insider information that you and a few others are privy to, rather then an official statement made by LTC to the general public. It isn't even listed on their website or telecharge.
She's had issues with her voice for many months now on and off, so I wouldn't be surprised. Plus, Rigg calling her out was pretty awful as well. Although, they probably would have made that announcement so ticket buyers could be in the know.
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How ironic would it be if Benanti ended up doing it as a replacement? However, she was very public about the fact the Bartlett Sher did not demonstrate any real interest in casting her when she went out for the role. That said, Sher might have always pretty much planned on having Ambrose do it, and had other ladies audition just do "make sure." Now that Ambrose is leaving, and he's seen the wildly mixed response to her, he (and the producers) may want to go with a more sure-fire, "safe" choice like Benanti.