I assume a lot of people roll their own containers and use STL algorithms, which makes a lot of sense if you need control over allocations but still want to utilise the STL.
Yes. I think this is so common that Rust, positioning itself as an option for a systems programming language, has the #![no_std] macro to not link with the full standard library.
anecdotally in the last ten years I haven't seen a single C++ codebase not using the stl, except arduino-level stuff