regarding the primary focus of this report, the comment #0 code no longer runs in Matlab:

matlab 2022a:

>> r = randn(4,4);
>> subsasgn(r, struct('type', '()', 'subs', {{2, 2}}), 0)
Error using indexing
SUBSASGN must be called with an output.

Not interested in digging through a decade worth or release notes to see if it's mentioned, but it appears that sometime over the past decade this undocumented behavior changed.

Now Octave still allows a subsasgn without an output, and simply puts the result in ans, leaving r unchanged:

>> r = randn(4,4);
>> subsasgn(r, struct('type', '()', 'subs', {{2, 2}}), 0)
ans =

   1.8659   0.2015  -1.9407   0.1915
   0.3151        0  -0.1796  -0.8894
  -1.1896  -0.5194   1.1625   1.0586
   0.2087  -1.3228  -0.3853  -0.2635

>> who
Variables visible from the current scope:

ans  r

>> ans
ans =

   1.8659   0.2015  -1.9407   0.1915
   0.3151        0  -0.1796  -0.8894
  -1.1896  -0.5194   1.1625   1.0586
   0.2087  -1.3228  -0.3853  -0.2635

>> r
r =

   1.8659   0.2015  -1.9407   0.1915
   0.3151   0.1767  -0.1796  -0.8894
  -1.1896  -0.5194   1.1625   1.0586
   0.2087  -1.3228  -0.3853  -0.2635

of the two speed examples given in comment #0, Matlab no longer allows the comparison, but both 'slow' versions run almost instantaneously in both Matlab and Octave. Don't know if this is because both programs have made these optimizations already, or if things are just faster 11 years later. 

In any case, I'm going to change the category to Wish/Feature request.  It should probably be closed as Wont Fix unless someone  can verify the code optimization still needs to be addressed.

It is not only a Matlab compatibility issue, but apparently an inconsistency with Octave documentation.

octave:1> help subsasgn
[...]
          val = magic (3);
          idx.type = "()";
          idx.subs = {":", 1:2};
          subsasgn (val, idx, 0)
               =>  [ 0   0   6
                     0   0   7
                     0   0   2 ]

     Note that this is the same as writing 'val(:, 1:2) = 0'.

However, last sentence is not correct (val is not assigned after subsasgn call). Example that shows it:

octave:1> val = magic (3);
octave:2> idx.type = "()";
octave:3> idx.subs = {":", 1:2};
octave:4>
octave:4> subsasgn (val, idx, 0)
ans =

   0   0   6
   0   0   7
   0   0   2

octave:5> val
val =

   8   1   6
   3   5   7
   4   9   2

octave:6> val(:, 1:2) = 0
val =

   0   0   6
   0   0   7
   0   0   2

octave:7> val
val =

   0   0   6
   0   0   7
   0   0   2

If I understand it correctly, documentation should be modified/clarified or subsasgn implementation modified.

I can't confirm whether this is the behavior in Matlab or not, but confirmed that subsasgn does not modify the value argument in place. I don't think any Octave function modifies the value of its argument.

Can someone confirm whether this is the behavior in a current version of Matlab?