On July 1, 2023, Google announced they will be deprecating Universal Analytics, which means they won’t be processing any new hits after that date.. On October 1 of the same year, Universal Analytics 360 properties will stop processing hits as well.
In the announcement, Russell Ketchum, director, product management at Google, said,
“Universal Analytics was built for a generation of online measurement that was anchored in the desktop web, independent sessions and more easily observable data from cookies. This measurement methodology is quickly becoming obsolete.”
Any processed data that Universal Analytics has will be stored for at least six months after the cutoff dates.
With these dates in mind, you will want to set up your GA4 properties well before any of the dates happen so things will be as seamless as possible. This will also allow GA4 to track the metrics you care about so you will have as much historical data as possible.
Source – Search Engine Land