Private fields are initially landed with LLInt and partial JIT baseline support via op_put_by_val_direct. The extra semantics are not handled properly in the generic JIT stubs when IC support is given up on, and are also not supported in optimized code.
It was handled by https://bugs.webkit.org/show_bug.cgi?id=213372 *** This bug has been marked as a duplicate of bug 213372 ***