If you were to acquire one of the CZ 858s that would be deemed prohibited—because not all will—should this legislation pass as written, on July 5 nothing would happen. However, once the law passes and comes into force, and there is a requirement for you to attempt to register that firearm, in declaring to the registrar of firearms that you acquired it post June 30, you would be deemed to be ineligible to hold that firearm. The firearm would be prohibited, and you would not be able to register it.
On June 7th, 2018. See this statement in context.