https://snmetamorph.github.io/PrimeXT/docs/eng/modding_introduction/

absolutely b(i)ased introduction to Half-Life modding, calling Xash release "the most important event in modding history", by @snmetamorph

I honestly agree. Even if you have problems using Xash code, because of legal reasons, or because you're just Valve (I know my followers here ;)), it still explains the behavior of many GoldSrc API functions, that previously were barely documented.

Even if the modder don't want to support Xash, it's still possible to leverage the fact that the engine is completely buildable from the source code, meaning they can easily connect a debugger or add modern run-time instrumentation tools like AddressSantizer and UndefinedBehaviorSanitizer.