I find it kind of interesting, can anyone try to get it working? i wanna try it out to see what it could have possibly been like, considering the only thing left is the bossbattle submode.
I don't think that code is compiled into the game. You are best asking programmers familiar with the TF2 code, which you will find on Discord servers for TF2-based mods like TF2 Classic and Open Fortress, and the Allied Modders Discord.
The code is indeed not compiled into the game, being hidden within STAGING_ONLY blocks of code. You would indeed have to port the entire mode to a SourceMod server plugin to test it and nobody has done this.
Most of the aforementioned Discords are not the best start, other than the AlliedMods discord. I personally would start on the AlliedMods discord for this.
You could also compile your own build of the code leak with the staging blocks enabled which other people have done in the past. If you are going to go this route, then I would start with the TF2 mod Discords.
Here's a preview of some of the boss functionality:
If this is your first time logging in after the migration (Feb 8, 2022), you must reset your password to log in. Follow this guide if you're having trouble