I’m not aware of any specific information regarding a plan for “all of Milan to be ‘zone 30’ from January 1, 2024,” and I do not have the capability to find articles from any external sources, However, I can tell you generally what it means to establish a “zone 30.”
This is typically done in densely populated urban areas or areas with a high concentration of pedestrians and bicycles to improve safety and reduce the likelihood of accidents. These zones are also often established in areas where there are many schools, parks, or other places where children and vulnerable road users may be present. The goal of a zone 30 is to reduce the severity of accidents and injuries, if they occur, and make it easier for people to walk, bike, and drive safely in these areas.
It is possible for the City Council of Milan to vote and approve such an agenda, as many Cities have already established zone 30 areas, but as I have no information about it, I am unable to confirm if this is the case or not. If this is true, it could be that the decision is made to improve the safety of the city, reduce air pollution and encourage sustainable mobility.