There was no recent change at all.
there was a recent question so we looked into it (again).
This subject was discussed several times in the past and remains debatable.
The reason for a limitation, as Jonni already said, is to protect corporations against overspending that does nothing to improve their revenue.
The output quality depends on the quality of the used products and the upgrade level of the corporation.
If you could use input quality 400, the output quality could go much higher and nobody will ever buy your products or even worse, the trading system might crate a compromise price to find a deal and your revenue will end up much lower.
The best solution to the problem of extremely high quality products proved to be the prevention of escalating quality that causes heavy losses to corporations and countries.
We will look into it again and see if the settings are good in all cases and may come with some different results in specific cases.
The general idea however remains the same and it is unchanged from day one this game is running. It was tuned several times in the past years. Not recently, we saw comments in the source, dated to 2011 and 2012.
I do not think anyone could claim the last word here.
There are major differences in the way the game economy works, compared to the real one,
One of them is the rigidity of the use of raw materials.
In the real world there are many alternatives and millions of products.
Another is, that in the real world, there is a market for a wider range of qualities and there are more, and different factors involved in the quality of products.