struct2cell seems to be lowest level still extant, yes. <struct2array#answer_403725> gives some history(*) of its evolution from the Signal Processing Toolbox to main product to now apparently red-haired stepchild.
I suppose that since a struct isn't required to be directly convertible to an array given the different field content possible, TMW decided it wasn't a generally-enough-applicable function to keep so they abandoned it.
(*) I don't know when I've used it, if ever; I also tend to avoid struct when can, generally using the table for heterogenous data instead, so hadn't ever noticed anything about it.
I'm still using R2020b here; there it is still extant, but the doc page is just the help text one-liner. I dunno, that may have been the extent always on that...
>> which -all struct2array
C:\ML_R2020b\toolbox\shared\measure\struct2array.m
>> which -all struct2cell
built-in (C:\ML_R2020b\toolbox\matlab\datatypes\@struct\struct2cell) % struct method
>>
All struct2array is is the catenation of the output from struct2cell into an array -- which may/may not always work...
>> type struct2array
function a = struct2array(s)
%STRUCT2ARRAY Convert structure with doubles to an array.
...dpb snip...
% Convert structure to cell
c = struct2cell(s);
% Construct an array
a = [c{:}];
>>
