This never got an answer, so for anyone who lands here from a search:
Wix Stores has no built-in implementation of the 30-day-lowest-price requirement. The compare-at (“was”) price field is not it — you can type any number into that field, and nothing records that the item was ever sold at that price, which is exactly what the rule asks you to be able to point at.
What Article 6a wants is the lowest price you actually applied during the 30 days before the reduction, and since C-330/23 the discount percentage has to be calculated against that figure rather than against yesterday’s price. Two things trip implementations, in case you build it yourself: a price you set before the window still counts, because the rule asks which price was applied during the window and not which price changed inside it; and a variant you delete has to stop counting, or the product page keeps quoting a low that nobody can reach.
On the “is there an app” part — I built one, and to be clear it is mine: Omnibus 30-Day Lowest Price | Wix App Market | Wix.com. It records the store’s own price changes and shows the line only while a product is genuinely below its reference; that line is free on every product. Either way, happy to answer questions about the rule itself.