Why?

F3 is "search again" almost universally, why would it be "Show status bar" in gwenview?

This should be closed (by @vpilo presumably, Phabricator doesn't let me do it), because this has been committed back in May already. Changing or removing the shortcut again would potentially affect users, since this is shipping in various distributions since 17.04.2.

Regarding the "why": I'm not the author/committer, but I find F3 sensible and also useful, especially for Gwenview in fullscreen mode. Currently we have the following list of F-key shortcuts:

F1 Gwenview Handbook
F2 Rename…
F3 Show Statusbar
F4 Sidebar
F5 Reload
F6 Edit Location
F7 Copy To…
F8 Move To…
F9 Link To…
F10
F11 Full Screen Mode
F12

Placing "Show Statusbar" right next to "Sidebar" seems like a good fit looking at the only two other F-keys still available. Also, I'm not aware of any work introducing a search functionality to Gwenview or a common standard action/shortcut for hiding the statusbar.