grammar - What is the nuance when は directly follows a verb in plain form?

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:
(At least I suspect that it's not the "but" kind of が. Feels like the subject particle for some reason.)

巡り逢ったが　運の尽き

(刀と鞘 - ALI PROJECT)

Answer

This construct was common in classical Japanese, but now it is archaic or poetic. In classical Japanese, the attributive form of conjugating words can be directly followed by particles which attach to nouns (without inserting の). 目指すは would become 目指すのは in modern Japanese, 吹きやまぬは would become 吹きやまぬのは or 吹きやまないのは, and so on.