Is it

Honey

The black and golds are

Bees

And "And left to swim among the shrooms until it's time to stand,

And in that time my power grows as was the hand's intent,

Til even mighty fall like fools when my power is spent" might refer to

Mead, which is made from fermented honey

Is it

Mead

"My source is borne upon the wing of valkyries of gold,

They work their magic summer long then winter through the cold,"

Bees make honey

"Once the black and golds are done I'm plucked with careful hand,"

Honey collected by beekeepers

"And left to swim among the shrooms until it's time to stand,

And in that time my power grows as was the hand's intent,"

The honey is left to ferment into mead

"Til even mighty fall like fools when my power is spent,"

Mead gets people drunk

"Though fewer know me by the days, some think I'm in the past,

My history is far from gone, it stretches wide and vast."

Mead was more popular historically than in the modern age