Where IPYNB comes from
Fernando Pérez started IPython in 2001; the browser-based notebook arrived in 2011 and was split out as Project Jupyter in 2014 to reflect that it had outgrown Python. The nbformat schema has been stable at version 4 since 2015. Notebooks became the default medium for data science and machine-learning work, and are now rendered directly by GitHub, GitLab, Kaggle, Google Colab and every major cloud platform.