Thank you! Separately, but related: fuck you, Google! (every time I have to deal with protobuf in C++, I curse Google and their "we don't give a shit about signed vs unsigned comparisons").
Yeah, some warnings are turned off, but not as many as you'd think, and usually for good reasons, which also includes deliberate design decisions. For example, we don't care about pre-C11 compatibility (because we won't build for pre-C11), so that warning is off. We also like 0-sized arrays, so that warning is off as well.
It's a moving target, because compiler engineers add new warnings over time. Adapting the new compiler means taking care of the new warnings. There's almost always a way to do so instead of turning a new warning off.
By keeping your own project warning-free in your environment, you are doing a service to everyone.
edit to add: and yes `-isystem` is absolutely a useful tool. If memory serves, though, it doesn't protect from macro or template expansions, though.