For an own campaign, preserve the approved version, stock forecast, supplier commitments, advertising plan, actual reach and sales. Add every amendment, stop instruction, customer communication and treatment of existing orders.
A competitor should preserve the full advertisement with date, URL, price, period and visible qualifications. A single “unavailable” screenshot does not establish the whole case. A factual ordering attempt, the offered alternative and repetition across channels may be relevant.
The canonical German article on the UWG blacklist places all automatically prohibited consumer practices in context. This article remains limited to bait advertising, availability and a planned switch to other goods.