"All lives matter" is a natural English sentence expressing the idea that every person's life matters. This is not a particularly controversial idea.
I recognize that it's also a non-neutral political slogan, and that modern speech conventions require people to avoid saying things that sound like controversial political slogans. But that's precisely the problem! I have to keep up to date with all major partisan controversies, a task I generally find quite miserable and pointless, in order to know which new phrases I should avoid!
