Jemalloc was removed in November 2018 [1] and didn't land in stable where it defaulted to the system allocator until Rust 1.32 released in January 2019 [2]
At the time in 2018, however, I was using Linux but now use Windows. I just now learned that Rust used the system allocator in Windows longer than for Linux [2] where it was still using jemalloc for the majority of 2018.