The per unit price tends to be pretty small print though so you need to actively look for it which means that deceiving customers via shrinkflation is still possible as most repeat customers will only look at the physical packaging size and the sticker price which can both stay the same while decreasing fill size and increasing per unit size accordingly.

And you also sometimes get different units being used for related products. And of course then there are cases the the amount of product is not the relevant unit for price comparison because different competitor formulas need different amounts for the same effect. Still plenty of tricks that unscrupulous companies can use in the EU.

The US also commonly has per-unit prices on the shelf tags. I think that practice started in the 1970s after some studies suggested it was beneficial.