You can use flour 2 years out of date.
Flour doesn't go bad in the way that it will make you sick and so flour is safe to use 2 years or even longer past the best by date.
However as flour gets older it starts to lose it's quality so it may not result in a great recipe that you make with the flour.
As long as the flour has no bugs or other things in it that would make it bad, the out of date flour is safe to use.
White flour is fine to use no matter how old it is, although whole wheat flour and other whole grain flours can take on a metallic or soapy odor as it ages.
You can bake with out of date flour, although the taste may be slightly different.
I've used white flour that was 3 years out of date and the recipe turned out just fine.
The best by dates on flour packaging doesn't mean the flour has gone bad by that date, but instead it's a date that the manufacturer of the lour suggests you use it by, as the flour as at it's best quality up until the use by date date or best by date.
Using out of date or expired flour won't harm you like other foods could if they were out of date.
Over time, flour can lose it's freshness, which can lead to a decline in the taste, texture and sometimes can make some baked goods not rise properly.
Even some certain types of other food are still good and safe to eat when it's past it's best by date.