So it's just as useful as when your stack area ends with a page that will segfault on access, or your CPU will raise an interrupt if stack pointer goes beyond a particular address?

It's not safe though because throwing an exception, panicking, etc, is still a denial of service. It's just more deterministic than silently overwriting the heap instead. If the program is critical then you need to be able to statically prove the full size of the stack, which you can do with C and C++ with the right tools and restrictions.

A segfault is considered safe if you're talking about functional safety because it results in a return to a defined safe state (RTDSS).

If a segfault leads to some other state you do not deem "safe", such as a single program gating access to a valuable asset with a default fail state of "allow", you just have a fundamental design flaw in your system. The safety problem is you or your AI agent, not the segfault.

Deterministic, well-defined behavior is inherently safer than undefined behavior. It allows you to diagnose the problem and fix it. UB emphatically does not, and I don't dare to think of how many millions of person-hours are wasted every year dealing with the results.