It was relatively popular on here a few months ago, a kind of medium-style site but the twist was you could run code examples, edit variables and indeed edit/fork the whole source. Often used for explanations of programming or statistical concepts. For example I saw one for COVID-19 where you could adjust variables like R to see a live update of the drawn curve, and run simulations of people interacting with various rates of transmission etc. Googling various related terms has not shed any light...
