The tour is now back in the top bar drop-down menu.
See the earlier version of my answer below, where SE said this was an unintentional change which they were fixing. It has now been fixed (at least on our site).
As dim kindly commented, this has been recognised as a regression on Meta.SE:
"The tour link has disappeared from the help menu in the top bar"
I'll make that information more visible by posting it as this answer.
The SE staff member (actually VP) Philippe said on that page:
This was an unintentional regression. We're working on getting a fix pushed out right now. Will update when completed.
Here's a screenshot of that Meta.SE page currently (click for the full size version):