I’m not a lawyer, so it’s hard for me to precisely judge the limits of terms of service.
Is it possible to put GPL code in Observable notebooks, without breaching the license? That is, anyone else redistributing the same code (e.g. by forking the notebook and modifying it) would be restricted to releasing those modifications under the same terms, etc.
Yes! You are welcome to specify a license for your notebook, including GPL.
Our terms of service are based on GitHub’s open-source site policies. Specifying a license on an Observable notebook is analogous to specifying a license on a GitHub repository. Regardless of license, you grant other Observable users permission to view and fork your notebook. And in the case of a GPL’d notebook, a redistribution (such as a fork) of the GPL’d notebook would need to be released by the fork author under a GPL-compatible license.