Shell shock has a long history, covered in depth here

In brief, it may appear in the Book of Job, and Greek Historian Herotodus probably alluded to it. Homer appears to have mentioned it. For sure it shows up in Shakespeare's King Henry the IV. MacBeth has famous scenes of trauma reenactment ("out spot out").

The course of World War II was altered when General Patton did not believe in shell shock but rather slapped a soldier who was suffering from it.  Society recognized the problem and General Patton was relieved of duty. Had this not happened, many people believe that the war in the European Theater could have come to an end in August 1944 during the Battle of  Battle of Falaise pocket , when Patton had the Axis surrounded but General Montgomery ordered Patton to let the Axis retreat.

It entered the DSM, as part of DSM-III 61 in 1980,  thousands of years after it was known to man. Also, note that Tourette syndrome too 95 years to be included in DSM-5, after it's discovery in 1885. It was first included in DSM-IV.

Because of this, it is not correct to say "parental alienation does not exist because it is not in DSM-5".   Much more importantly, besides, it is in DSM 5 according to the authors of DSM-5 who published a paper to explain that it is indeed in DSM-5.