The long-time metric of Bounce Rate in Universal Analytics will not be carried over to Google Analytics 4 (GA4). Bounce Rate is defined as the percentage of single-page sessions in which there was no interaction with the page. A bounced session has a session duration of 0 seconds.
In June 2020 Apple announced plans at their Worldwide Developers Conference (WWDC) to have new privacy measures in place with the release of IOS14 on mobile devices. There has been a lot of confusion around what these new measures are and how they are going to affect tracking platforms such as Google Analytics as well as Digital Marketing initiatives/ advertising.
Recently, you may have received an email about connecting your new GA4 Property to your Universal Analytics Property.