The handling of caption has never changed in latex: the code for \caption and \@caption has literally not changed at all since we started the current svn in 2008, I didn't check back beyond that but essentially it hasn't changed since latex2.09, 30 years ago.
The caption is just set where it appears in the source file. This is really the only possibility if you allow authors to have multiple captions in the same table or position the captions to the side in minipage or other formatting rules.
Some packages and classes use a more controlled format where only one caption is allowed and it is saved by the float mechanism rather than being typeset and then positioned according to some document options. Notably any document using the float package (or a class that uses float) has the possibility of the float mechanism taking over caption positioning via that package's \restylefloat command.
But as Christian mentioned in comments, if you want to look at old copies of latex for other reasons, then http://www.latex-project.org/get/#historic has some links.
latex.ltx(or maybelatex.tex). – Przemysław Scherwentke Oct 09 '17 at 12:47