Just that some aliases exist for consistency doesn’t mean that the
   whole package is filled with aliases.

Nobody claimed that, quite the opposite.  The main flaw of s.el is
still that it is _mainly_ trivial wrappers.

A function for finding the common suffix seems like a useful addition,
that is very concrete and can most surley be added to e.g. subr-x.el
or subr.el as maybe 'string-common-suffix STRING1 STRING' or something
similar, and implemented using `compare-strings'

It would be interesting to hear how people use this function, is it as
a predicate (where string-suffix-p on both strings would suffice) or
do you use the returned value which seems uninteresting?