A few countries did that, but those extensions were mostly predicated on a 50 years copyright (the "morts pour la france" extension also is, it's 30 years tacked onto the original 50). When copyright got normalised to 70 by EU rules (or separately extended before EU or EU entry), these extensions became moot in EU countries. IIRC Russia is the only country where war copyright extensions still apply, as a 4 years extension was specifically added when they rewrote their copyright laws in 1993.