Were Holes Or Plugs Ever Collected For Assay Or Taxes?

That's an interesting question. While I haven't done any research on the subject, I would think that "yes", they would keep the metal. It could be melted down and re-used. I'm curious what others say. Thanks for the question!

No.

Freshly-minted coins put aside for assay (official government testing, such as for Britain's "Trial of the Pyx") would have either been non-destructively tested (i.e. weighed), or would have been tested to destruction (i.e. broken clean in two). What they would not do, is punch a hole in a coin, then release that coin back out into circulation. That holed coin is "damaged" and no longer fit for use, as it no longer contains the specified quantity of silver.

Once in circulation, coins get damaged in all sorts of ways. I suppose back when silver dollars were routinely cut into pieces to make change, a tax collector who didn't have any small change to give back might have requested a larger coin be cut. Can't say I've heard of it happening. And they'd never punch out a little piece to create a holed coin - they'd cut a coin into pie-shaped wedges instead. The round hole would be too difficult to make, compared with a straight cut.

A coin with a hole punched in it has almost always had the hole punched for aesthetic reasons - turning it into a piece of jewellery, most commonly.

Sometimes, if a merchant was certain a coin was counterfeit, they'd nail it to a wall or doorpost, as a way of getting it out of circulation (and to remind themselves to be more careful when accepting coins from customers). The nails used back then were usually square, creating a square hole.