update: patch #10314 was pushed to core which adds matlab compatible dim handling, including the "all" option, for mean, median, var, and std. no change to other mfiles yet nor any compiled functions in comment #3 list.

so what is the equivalent in the compiled functions?

Changed the severity to "Minor" since there is an easy workaround that is supported by both Matlab and Octave.

in v2018b, Matlab added the 'all' option for a number of functions. 

This was mentioned related to another missing option in: https://savannah.gnu.org/bugs/?func=detailitem&item_id=58089#comment1

which states it applies to "all, any, bounds, max, min, mean, median, mode, prod, std, and var".

This was noticed on the mailing list regarding the sum function when a new user tried to port over some code.

while bug #58089 was about a the option where the functions take a vector dimensions for multiple summation, the "all" function should be fairly trivial to implement as:

if "all" --> A = A(:), continue sum operation

At first glance at the function list it looks like this approach should apply to them all.