Neither is almost anything, but realistically it's available on every mainstream and even obscure OS (might not be named the same thing). If you don't have it, a lot of things (plugins, etc.) become a lot harder to do.
It only becomes hard to add native-code plugins to your closed source project. If you're open-source you just build it into the source code and recompile it (or add it via the command line to your compiler and it will become #if-included in).
Also, most programs don't have a need for plugins and those that do, the plugin interface is a very small part of the design and certainly doesn't factor into the language you use to write the code, or whether unit tests are a central feature of the language, or something you can choose to ignore to your later regret.
And if that wasn't enough, LoadLibrary isn't an eval, and isn't really equivalent to adding dynamic code to the codebase. It's dynamically adding more static code, which is quite different. Nobody in their right mind generates the library that they want out of a string of data to generate a library that they then LoadLibrary in.