Nothing wrong with that if the toy meets all of the functional and non-functional requirements.
If you find offense to this, the easiest way to mitigate is with process and practice: sandbox code goes into a dedicated "Sandbox" mono-repo and if it's suitable for production, you rebuild it appropriately in a production repo.
Exactly. That's why I don't build the toy anymore: Too many broken promises of "Yes, we won't put it into production until it's ready", and then my team is left maintaining a system in production that had no business of ever being in production.
If you find offense to this, the easiest way to mitigate is with process and practice: sandbox code goes into a dedicated "Sandbox" mono-repo and if it's suitable for production, you rebuild it appropriately in a production repo.