That would be because 32-bit support is not gone. The bytecode compiler continues to work on 32-bit systems, we conduct best-effort testing to keep the ecosystem working on these platforms, and almost all packages on (e.g.) 32-bit Raspbian will continue to work with OCaml 5 – just more slowly.
32-bit native code generation support is gone in OCaml 5.0, with no plans to bring it back.