Yes, MSVCRT.DLL has been included with Windows since forever. But it isn't part of the Windows system. It is just an interface DLL for programs written in C to be able to use the C standard library functions when running in Windows. If you want to read the documentation for MSVCRT you have to go to the C related pages to see how to use it. The normal Windows documentation doesn't address it.
msvcrt was "promoted" to "System DLL" so it's actually part of Windows now (I think around W2k era). What this means is that it's actually required for your system to work, as other system DLLs presumably depend on it nowadays.
But that's only msvcrt.dll -- all the other ones are redistributables still.