Has C++ Always Allowed Using A Variable for Array Size? [duplicate]

Answer 1

Was anything in the language updated to allow this

No. Variable length arrays (aka. VLAs) are a compiler specific extension.

The c++ standard never allowed this (unlike the c99 standard does so contrarily).

Answer 2

The C++ Standard does not support variable length arrays though some compilers can have their own language extensions that allow to use VLAs in a C++ program.

Thus this code snippet

int arraySize;
cin >> arraySize;

int array[arraySize];

is not C++ compliant.

Use instead the standard C++ class std::vector.

As for C then according to the C Standard implementations may conditionally support VLAs.

You can check whether an implementation supports VLAs. From the C Standard (6.10.8.3 Conditional feature macros)

1 The following macro names are conditionally defined by the implementation:

__STDC_NO_VLA__

The integer constant 1, intended to indicate that the implementation does not support variable length arrays or variably modified types.