You changed the vector from a mutable one to an immutable one. That isn't the same code. What if you need a mutable vector?

Also, consider this:

      std::observer_ptr<std::string const> const foo() {
          std::vector<std::string const> const strings { "Hello" };
          return std::observer_ptr(&strings[0]);
      }

      std::cout << foo(); // use after free, even with const!

This is how these discussions always go: someone presents an example of use-after-free in C++, someone else says "that's contrived"/"not real C++", and the discussion continues indefinitely. At some point we are going to just start debating "does anyone actually write use-after-frees in modern C++?", which is also a question we have empirical answers to by way of vulnerability databases and bug trackers, and the results don't look good for C++ there either.

If your contention is that observer_ptr is completely safe, then we could have easily added it to Rust. We didn't for a reason: it's unsafe in Rust and unsafe in C++.