According to a discussion thread I'm on in the trailblazer community (
https://trailblazers.salesforce.com/0D54S00000A7Hjc) the following new features are available for use with Scratch Orgs:
* PublishExpBuilderBasedSna
* PublishAuraExpBuilderBasedSna
I noted that the JSON schema highlighting indicates that the schema for the scratch def doesn't recognize at least the second of these:
Having updated the sfdx-project.json's API version to 52.0 didn't change this behaviour.