There is no EU legislation that governs suffocation warnings on polybags or on plastic bags generally. The European Committee for Standardization sets technical standards for plastic bags, but this does not extend to suffocation warnings.
What Amazon follows is the US Regulation which is not a requirement in the Europe. However, it is a good practice to have a Warning printed in polybags. The only EU Regulation that specifies some sort of safety which could extend to plastic bags, is The European Toy Safety Directive Council Directive of 3 May 1988 on the approximation of laws of the Member States concerning the safety of toys (Directive 88/378/EEC). Article 1, Section II of Annex II of this Directive states that: “Toys and their parts and the packaging in which they are contained for retail sale must not present risk of strangulation or suffocation.”