Data retention in GA4 refers to the period of time that user-level and event-level data is stored in your GA4 property before it is automatically deleted.
The default is set to 2 months only. If the user does not return to our site within 2 months, the data about him will be deleted, in the 3rd month GA4 will consider him as a new user.
In Universal Analytics it was defaulted to 14 months.
You can change it by going to Admin > Property > Data Settings > Data Retention >> Change to “14 months” (maximum)
Leave the “Reset user data on new activity” option enabled.
