postInstallScript marked as not valid in sfdx-project.json

[Phil W]

This element is necessary for 2GP managed packages and does work, but is marked as not allowed by the IDE:

‌

[Scott Wells]

I'll add it for the next build. The underlying JSON schema files aren't very well-maintained in their original location, so I generally have to update them manually for any new attributes, changes, etc.

[Scott Wells]

This should be long resolved. Let me know if you're not seeing it available now.