I went to this website to see the differences between Rails 5.0.0 and Rails 5.1.1
Why does 5.1.1 not anymore include:
Here's the commit where it was removed: Setup default session store internally, no longer through an application initializer
In summary, new apps don't have that initializer, instead the session store is set to cookie store by default. i.e. the same as the value that used to be specified in the generated version of that initializer.