All entries from your feeds are stored in a local database on your device. The size of this database grows in time depending on what feeds you subscribed to. To avoid going out of storage space, a feed aggregator needs to delete older entries.
The old app deletes older entries when the count of entries exceeds the configured limit.
The new app uses a different strategy where entries are deleted once they exceed the configured age covered by the new cleanup mode option.
The default cleanup mode is set to 1 month but you can change it in the update settings, or specify a different cleanup mode for each feed in the feed settings.