Since the import assertion syntax (e.g. assert {type: 'json'}) is mandatory in the latest Node.js versions, I'm looking for a way to be able to write this new syntax, but I don't want to add Babel just to be able to use the Babel ESLint parser which understands it.
Could this be added by a different ESLint plugin?
As of this writing, the Node.js core code itself is configured to use @babel/eslint-parser specifically for this. It is unlikely that there is an easy and reliable way to do it using a parser shipped with ESLint (at this time). As of this writing, the specification is Stage 3. ESLint has a policy of only adding Stage 4 syntax. Additionally, Node.js docs and a runtime warning make it clear that this is an experimental API. Installing @babel/eslint-parser is the reasonable solution if you're using an experimental API.