Then you missed the continuation of that thread which clearly states
that the problem was fixed in 1991.

-----

The 89 was the first of the C4 line, while the C2 was the old body style
for that year.  In 90, the new 964 C2 was introduced without head
gaskets on the 3.6l engine.  The engine ran slightly hotter than
predicted, so the heads would warp a little and cause oil to seep and
leak.  This problem was ***fixed in 1991*** and a dealer procedure was
in place for any cars exhibiting this warping.  Not all cars were
affected, but all were susceptible. I think it was an owners choice to
have the procedure done.   The reasoning (then) was that if was not
manifesting itself after several tens of thousands of miles, the chances
of it happening were slim, but it does still happen [which means that it
needed to be done!].  So, the trick is, ask.  If it had been fixed, then
it is not an issue.  If it has not been fixed, it may manifest itself,
or it may not, but it should drop the price accordingly.

The dual-mass flywheel was another major problem.  This only affected
the 1990 cars.  If the original faulty DMF was left, this may manifest
clutch problems, but it is the flywheel that caused it.  The fix was to
fit the C4s with the DMF from the Turbo and use LUK as the new supplier
for the C2 cars.

The dual-distributer system was produced without a vent to exhaust the
ozone created by the spark and this would rapidly deteriorate the
synthetic rubber belt that kept the distributors in sync.  Bad stuff if
it broke! [and a very easy fix]

So, if these fixes were done, then these cars are great!  Like I had
mentioned, the knock on them means that awesome deals are out there for
folks wanting a newer 911 cheap.