Like jaquesm says, I just bought plywood at home depot and it came in 6.45mm thickness, 24"x48". I don't know how you get 6.45mm either in metric or imperial, but it's clearly not "sized in metric".
No non-hardwood plywood, even 1/4" plywood, is oversized, and 6.45mm would be oversized at .2539 inches. If it was actual 1/4" (hardwood plywood), it would be 6.4mm per APA (american plywood association) standards.
Outside of hardwood plywood, thicknesses are really done in metric.
1/4" nominal plywood nowadays is sized at "15/64" imperial, or in reality, 6mm.
1/2" nominal plywood is 12mm, or "15/32".
(the actual sizes are metric, but are close enough in tolerance to imperial /32 or /64 sizes that they work)
The only plywood you will find that is "actual" size in imperial is domestically produced hardwood plywood.
This is not a large percentage of produced plywood :)
Actual thickness you mean...
All construction sheet goods available in the US come in 4x8 sheets (drywall excepted, which I believe can be as long as 12').
Just because you can express "4ft by 8ft" as "Xmm by Ymm" (and that very well may be the official spec) doesn't make it 'metric' lumber in any meaninful way.
In any case, let me make this very simple:
All non-domestically produced (and most domestically produced) sheet goods are produced to metric sizes that are close to 4ft x 8ft x whatever thickness. They are whole number integer metric sizes. They are metric sheet goods. They are produced and measured metrically by every producer and exporter.
They get sold as imperial sizes in the US. However, the fact that they are close to some imperial size does not make them imperial sized lumber in any meaningful way.
If i take a bunch of things that were produced as squares, sell them as circles, and people keep calling them circles, that doesn't make them circles.
You can call a 4ft x 8ft x 1/2" piece of sheathing 4ft x 8ft x 1/2" if you like. But it's neither 4ft, nor 8ft, nor 1/2".
If you don't believe that sheet goods produced and measured metrically to whole number metric sizes are metric sized simply because someone in the US decides to call it something different, we'll have to simply agree to disagree as to whether they are metric.
I'm happy to agree that folks in the US don't work with the metric system intentionally, but they are working with it, whether they know it or not.
This whole discussion started because one of the premises given was that things like construction sheet goods (and lumber, but let's stay simple) would have to change for folks to be able to use the metric system in the US.
As i've shown, this is simply false, they just need to pretending they are imperial sizes, because the construction sheet goods are already done to metric sizes.