I've updated the Half-Life Singleplayer SDK 2.3 to compile with Visual Studio 2019: https://github.com/SamVanheer/halflife-2_3
I've also fixed all warnings.
Basic testing done with WON version 1.1.0.8 and Steam Half-Life. No further support will be provided, but if you're interested in working with this SDK version or if you need guidance on fixing up old SDKs you can use this as a reference. See the commits for how to fix issues encountered when upgrading to a newer version of Visual Studio.