Why does Cmd-I in Preview do something other than advertised? Somewhere after Lion (from which I upgraded to 10.10), Apple has changed the shortcut to show/hide the toolbar in Preview from ⌘-B to ⌘-I, at least it says so in the menu entries.
 But when actually using the shortcut, what pops up is (as one would expect), the info window. 


Can someone explain why they would change it (probably not a constructive question), or whether this is a bug and how to rectify this and get the old behaviour back? Can this be done with assigning custom keyboard shortcuts?
This is not a bug, although I can certainly see why it looks like one. That little symbol is a pipe ( | ), not the letter I. The pipe is the upper character on the backslash ( \ ) key.

So to type ⌘-|, the physical keys you need to press are: ⌘-Shift-\.

The difference between "|" and "I" was more pronounced back when Lucida Grande was the system font (in fact, this very post is set in Lucida Grande; you can see how the pipe is taller and thinner than the uppercase I). But OS X Yosemite switched the system font to Helvetica, and unfortunately the "|" and "I" now look basically identical.