This means that the property can no longer be used to view page views, traffic, backlinks and more for related blogs or websites.
The same thing applies to blogs or websites using AMP (Accelerated Mobile Pages). For this reason, all users of legacy properties from Google Analytics, recommended to migrate to Google Analytics V4 properties. At first it is recommended to follow the guide in the ‘Setup Assistant’.
This step is important for sharing data between properties, both old and new Analytics properties.