DrHappinessDMCA Notice
From Link:The original copyrighted work is Valve's game Team Fortress 2 ("TF2") https://store.steampowered.com/app/440/Team_Fortress_2/. The TF2 assets have been ported to Source 2 without permission and are being redistributed by Amper Software in a game mode for Facepunch's S@box. Facepunch has not licensed any Valve assets for S@box. The unauthorized porting and redistributing of Valve's assets without a license violates Valve's IP.
If this is to be believed (I have no idea how to verify this), but it seems that the reason this got a DMCA takedown notice is that it ported TF2 assets to Source 2 and redistributed it on s&box which doesn't have a license for using Valve assets. While I think its weird that TF2 assets being ported to Source 2 is a problem, there does seem to be some justification.
Kinda similar to when OpenFortress and things like that got DMCA'd for using leaked source code.
Edit: According to the TF:Source 2 team, they contacted Valve and got confirmation that this DMCA claim is legitimate.
I love how Valve is more worried about this shit, but is perfectly fine with doing nothing with this game unless the community itself makes 99 percent of the new content for them and then takes a profit from it with practically no real work.