As stated here, cart and checkout validations are currently in developer preview. Since I want to implement a custom cart validation for my shop, I need this feature in production. Is there any chance that this will become stable in the next month or so?
There is a published roadmap for validation functions here.
In case anyone comes across this question now: checkout and cart validation functions were declared stable in July 2023. They are currently only available for custom apps, for shops on the Plus plan. They will be available in the coming months for use in public apps as well.