Yes, it is a protocol limitation. The hostname length field in a SOCKS5 request is a single byte, whence the maximum length. As explained in the write-up, curl

Yes, it is a protocol limitation. The hostname length field in a SOCKS5 request is a single byte, whence the maximum length. As explained in the write-up, curl now returns an error in that specific condition, which seems a more reasonable thing to do.