I was about to defend Jerry and say "Well, maybe someone added the suffix to shrink before him, but he made it a thing", but after looking at Google Ngram Viewer, then yes, he is way off.
Usage of the word shrinkage topped in the 1930's and has been in steady decline during the Seinfeld years.
Usage of the word shrinkage topped in the 1930's and has been in steady decline during the Seinfeld years.
https://books.google.com/ngrams/graph?content=shrinkage&year...
Screenshot: https://imgur.com/a/6Esimrk