5
It seems like this is a remnant of (or reference to) older forms of Japanese. Is that all there is to it, or does it have special meaning?
Examples from songs:
歌声 笑い声 満ちる大空 目指すは憧れ
(DuDiDuWa*lalala - KOTOKO)
吹きやまぬは残り風 [...]
巡りゆくはこの心
(残り風 - いきものがかり)
And lastly, the same with が:
(A...