Disclaimer: I take supplemental PQQ regularly in amounts way above nanograms.
That being said…
It has been proposed, but there is not yet any consensus. Also, it’s not yet certain that the human body doesn’t produce what it needs. Although we know that Pyrroloquinoline quinone does interact with certain pathways, it’s also possible that they work without PQQ as a cofactor, rendering it non-essential.
The latest research seems to be finding new quinoproteins which are active in the enzyme which were previously thought to be PQQ-dependent:
Rat studies on a highly-refined PQQ-free diet showed that the rats didn’t grow as well, but it is not clear that the result was exclusively because of the lack of PQQ. Also, we are not rats - for example, rats maintain serum levels of vitamin C way above ours, even if they consume no vitamin C, because they produce it themselves. Most people, with no vitamin C, would get scurvy, and many would die.
This is an area still being researched. But regardless what we find on it being essential, the research is also suggesting that it has beneficial effects in our bodies. It could be one of those things that makes things ‘better,’ even if we don’t, strictly speaking, need it.
As others have noted, it’s ubiquitous in many kinds of foods, albeit in trace amounts - but you can hardly avoid getting plenty of nanograms of it:
Those are the amounts per gram of substance. No oats are on that list, but bread is (implied wheat) - then again, we don’t know if the wheat is the source, or the yeast.
The amounts of PQQ that have been used in studies which beneficial effects, however, are larger than you can easily get through foods, unless you’re a huge fan of natto for breakfast. The source for commercially produced PPQ is fermentation - it’s the bacteria that ferment soybeans into natto that produce a lot of PQQ.