Sven Co-op
Sven Co-op is a Half-Life mod by Daniel "Sven Viking" Fearon that was originally released in 1999. The game was later released on Steam in 2016 after gaining a GoldSource engine license. As part of their deal, the new standalone game was not permitted to allow third-part mod loading (https://discord.com/channels/818989352411463731/818992254680170507/1378800476212170855).
Sven Co-op 5.27 is planning to update studiomdl to load .jpg, .png and .tga textures and .fbx models out of the box (https://discord.com/channels/818989352411463731/818989352411463734/1353428629526417408). Sven Co-op 6.0 is planning to migrate the game and engine to 64-bit (https://discord.com/channels/818989352411463731/818992254680170507/1378799354252627978). A JavaScript API to replace the AngelScript API is also planned (https://discord.com/channels/818989352411463731/818992669413212170/1362520299999858829).
The Sven Co-op team was also given the Stukabat textures by Valve (https://discord.com/channels/170051548284583937/170051548284583937/1312329562654380043).
Reportedly the lead programmer David "Sniper" McDermott wanted to change the mod's name to Half-Life: Atomic in the 2020s (https://knockout.chat/thread/67539/46#post-2484393) although this did not end up happening.
Mirrors were tested in Sven Co-op by developer GeckonCZ using a feature from id Tech 1 - the r_mirrorlpha cvar.
r_mirrorlpha