that should have said patch #10314

You can use the octave_value function is_scalar_type() to determine if the argument is a scalar.  This is faster than checking whether the number of elements is 1.

I think improved input validation (! isscalar (dim)) would be a fine idea until this feature is actually implemented.

Changing category to Feature Request.  This is something that Matlab has added relatively recently as you point out.

If you really want to use this functionality in your code in the meantime it wouldn't be that hard to add an m-file to override sum().  Below is an example which is also attached as sum.m

function s = sum (x, dim)

  if (nargin < 1 || nargin > 2)
    print_usage ("sum");
  endif

  if (nargin == 1)
    s = builtin ("sum", x);
  else
    if (! isnumeric (dim) || ! isvector (dim))
      error ("sum: DIM must be a numeric scalar or vector");
    endif
    s = builtin ("sum", x, dim(1));
    for i = 2:numel (dim)
      s = builtin ("sum", s, dim(i));
    endfor
  endif

endfunction

(file #48747)

In Matlab, the dim parameter for sum can be a vector and not just a scalar, in order to sum across multiple dimensions:
https://www.mathworks.com/help/matlab/ref/sum.html#d120e1199287

Octave only implements the scalar version and silently ignores the other dimensions if they were provided. See this line in libinterp/corefcn/data.cc:

dim = args(1).int_value () - 1;

As a consequence, Octave does this:

octave:7> sum(ones(2,3),[1 2])
ans =
   2   2   2
octave:8> sum(ones(2,3),[1 Inf 2 NaN 0 -eps])
ans =
   2   2   2

while Matlab does:

>> sum(ones(2,3),[1 2])
ans =
     6
>> sum(ones(2,3),[1 Inf 2 NaN 0 -eps])
Error using sum
Dimension argument must be a positive integer scalar, a vector of unique positive integers, or 'all'.