There is no "DNS police" entity to force them, but just using a random domain will cause issues if that domain ever gets registered. First, you will begin leak

There is no "DNS police" entity to force them, but just using a random domain will cause issues if that domain ever gets registered. First, you will begin leaking internal data by means of DNS requests, server connections, maybe even internal emails - e.g. when a client attempts to connect to the intranet outside of VPN. Second, you obviously won't be able to communicate with the new owners of that domain.