Hi Arnaud thanks for your question.
The problems themselves can all be found in pdb.dieschwalbe.de. Type K='50' AND A='Plaksin' AND S='Problem' AND YEAR='1979'. All 50 are there numbered, with solutions. If you are typing from a phone make sure that you select the correct single-quote character: the vertical one.
The query reveals that they were published in the now-defunct magazine Problem, but it does not give the article text. I suggest Arnaud that you send your request to the Retro Mailing List firstname.lastname@example.org. This will put you in touch with all the current retro greats who would probably love to hear from you.
If that doesn’t work then let me know and I will approach someone individually.
EDIT: the 50x50 article is now available at https://www.janko.at/Retros/Articles/index.htm in Russian but I guess that google translate will work - and the diagrams are the not important part.