I feel like there's an unwritten "recently" in there. If you were to speak ill of Colonel Sanders, nobody would berate you for speaking ill of the dead. But when a CEO like Wojcicki, who made changes that were unpopular to the end-users (but helped turn YouTube into an actual profitable company) dies, it's considered very impolite to use that opportunity to bad-mouth decisions she made. When her son died earlier this year, that would've been a bad time to speak ill of her, as well, even though she herself was still alive.
A better phrase may be "Don't say things that will hurt the feelings of those who are grieving," but that doesn't roll off the tongue so easily.
She was a public figure. If millions of people around the world know your name then when you die, people will have things to say. Some will be good, some will be bad.
The custom about “not speaking ill of the dead” makes sense in a small IRL community, not for internationally famous people.
A better phrase may be "Don't say things that will hurt the feelings of those who are grieving," but that doesn't roll off the tongue so easily.