NetBSD has a compatibility layer to Linux' xattr API: https://ftp.netbsd.org/pub/NetBSD/NetBSD-current/src/sys/sys/xattr.h (w/ no access to the system namespace, whatever that's used for)

/*
 * Support for file system extended attributes.
 *
 * This provides an interface that is compatible with Linux's extended
 * attribute support. These calls operate on the same extended attributes
 * as <sys/extattr.h>, but access only the "user" namespace.
 */

The native mechanism is sys/extattr.h - this then also allows access to the system namespace.

Edit: In fact, it seems that NetBSD's extattr API is identical to FreeBSD's. So this might just be a matter of adding "netbsd" in xattr to the platform check.

Note: OpenBSD has neither ACLs nor xattrs.