That doesn’t seem to be something I’ve seen any of the core contributors acknowledge and is often overlooked. As a relatively inexperienced developer it seems like a bad thing on the face of it but the builds still succeed... what’s the tradeoff?
It also makes me wonder how compatible react apps will be, for example one build warning about a deprecated iOS feature I inherited actually causes a crash on iOS 12. We have over 400 warnings from the react code alone, how many of those are future land-mines? Quality development dictates your builds have few, if any, compiler warnings. Even if they are things you know will never be a problem you fix them or adjust your warnings so they don't potentially obscure other, more dangerous warnings.
That's a good rule for any professional development project, but when you are developing a shared class library for other developers to build professional products from, it's far more important. The React team's failure to clean warnings makes me question their actual ability and commitment.
As it happens, someone sent a PR the other day that fixed a bunch of these warnings just in time for the new release candidate we just cut this week. Please give 0.56.0-rc a try. If there's any warnings that still need to be addressed, send a PR or open and issue and ping me. This is something we want to actively clean up :)