[dataflow]

It's a great question (_LIBCPP_DEBUG was already a thing in libc++), and AFAIK the answer is supposedly "it used to be too costly to enable these in production with libc++, and it no longer is." I have no first-hand insight as to how accurate this perception is.

[alpire]

That's exactly right. We've had extra hardening enabled in tests, and that does catch many issues. But tests can't exercise every potential out-of-bounds issue, which is why enabling it prod enabled us to find & fix additional issues.