The new join package in Flux allows for more complex joins on multiple columns, enabling the performance of various types of joins including inner, full outer, left outer, right outer joins. The `join` function has been replaced by the `join.inner()` function and other specialized functions like `join.time()`, which join tables exclusively on the `_time` column. These new functions provide more flexibility and power for joining data in Flux, making it easier to handle late-arriving data and perform complex joins. The `join` package also allows for custom as functions, enabling users to create their own logic for handling null values and other edge cases. Additionally, the `fill` function with the `usePrevious` parameter ensures that previous values are not dropped when performing joins. The new join package is a significant improvement over the old join functionality in Flux, providing more power and flexibility for developers working with InfluxDB data.