It has been years since I last logged on to this forum in order to post something, however I feel obliged to step in in order to clear a few things up since the TS had initially discussed this with me on my Discord server.
With regards to encryption, most likely yes, as is the case with any early-2000 era Korean MMO. The good news is that it is often a simple cipher they pull out of their arse; often something barely more complicated than a simple foreach xor with some magic constant on the whole packet. Generally speaking, tracing forward from WSARecv until the decryption subroutine is found should be sufficient.
With regards to existing servers, there are no existing servers. In fact the last sign of any serious attempt to make a private server for Windslayer was back in late 2012. Any private server attempt now will essentially be an uphill battle. There are also no binary or source leaks, so one will have to grind at captured packets and the disassembly the old fashioned way. Suffice to say, their custom file formats still need to be figured out, which is always fun. Sarcasm implied.
Lastly with regards to the protocol documentation, I believe it's fair enough to say that it simply doesn't exist. Though speaking from experience with these early-2000 era Korean MMOs, we can assume that it's simple message-passing, usually exclusively TCP, with packet headers consisting of 2 bytes for the packet size followed by 2 bytes for the opcode, followed by per-opcode custom data that we can safely assume is never bitpacked. Encryption is usually only applied to the bytes after the custom header. Message structures may sometimes be unnecessarily recycled for more than one opcode in some of the games I've looked at / worked on. Needless to say, keeping in mind the fact that Korean MMO developers love to make their clients authoritative will help with deducing message structures from captured packets.
Just to be clear, I am in no way affiliated with the TS. I manage a Discord server for a server emulator project (for a completely unrelated game) that I head the development of. The TS simply hopped on and started talking about reviving Windslayer, and since he isn't well-versed in the technical aspects of server emulator development for legacy Korean MMOs, I decided to hop on here and clear some things up for him.
Before I go, I must admit that Spiritus in Terra is up there with some of the best black metal I've heard in years. Keep up the good work Kylotan, from a fellow developer/MMO networking enthusiast who's similarly a guitarist in an underground death metal band.