Many clients have asked us about year-on-year reporting as they have heard about GA4's retention limits. Unfortunately, GA4 does have a 14-month retention limit, but it's a confusing policy.
For example, if you intend to use the Explorations reporting feature in GA4, you will be limited to 14 months (or two, if you haven’t updated your data retention settings). However, if you just want to look at the standard reports you will not be limited. We can provide you with more insights about this.
However, if you are serious about your data, then we recommend using Google's data warehousing solution BigQuery to store your data and build custom reports.