It’s checked in debug modes (overflow is a runtime error). You can explicitly use the overflow operator (+%) if you know that’s the behavior you want.
So, to be safe people should build both Debug and Release?
It is also possible to selectively disable/enable safety for individual code blocks.
Worth noting that UB is also checked in ReleaseSafe mode. Related issue: https://github.com/ziglang/zig/issues/2301