sshkey.c:sshkey_private_to_fileblob disables support for RSA1 when SSH1 support has been disabled:
    switch (key->type) {
#ifdef WITH_SSH1
    case KEY_RSA1:
        return sshkey_private_rsa1_to_blob(key, blob,
            passphrase, comment);
#endif /* WITH_SSH1 */

but ssh-keygen.c will still include RSA1:
static void
do_gen_all_hostkeys(struct passwd *pw)
{
    struct {
        char *key_type;
        char *key_type_display;
        char *path;
    } key_types[] = {
        { "rsa1", "RSA1", _PATH_HOST_KEY_FILE },

which leads to runtime errors like:
  ssh-keygen: generating new host keys: RSA1 Saving key "/etc/ssh/ssh_host_key" failed: unknown or unsupported key type