In my opinion: JSON is not a programming language when it's used as a data transport format; however, it can represent code which an interpreter could evaluate as a program.

For example, a Lisp based on JSON:

https://github.com/kanaka/miniMAL

A program:

  ["+", 2, 3]

I suppose this is still not a proof that JSON is (or can be) a programming language, since it covers only the notation.

it certainly can be... https://owasp.org/www-pdf-archive/Marshaller_Deserialization...