We have rolled out a change to make the "Hide left navigation" preference global. This means that changing the preference on any site will change it for all sites.
We will also be making the "enable keyboard shortcuts" and "Disable top bar stickiness" global preferences as well. These will be coming shortly. Once live, if you have enabled one of these preferences in the past on any site, then that preference will become enabled on all sites.
At a later date we will consider adding an affordance to the UI to support pin/unpin of the left nav. That is not included in this change.