In MacOS Sierra and with the latest version of DejaVu Serif, neither the italicize font format menu item nor the system shortcut (Command-i) that calls it work any longer.
The menu item appears as grayed out when typing with the default (book) form of the typeface.
However, if you engage bold (command-B) first, the italicize shortcut then works. If you turn off bold first, you may no longer use the italicize shortcut to un-italicize text.
This seems to affect the RTF component of MacOS. Applications such as Pages that leverage a different system still work as expected with these shortcuts.