Midsole plugs are a design feature inserted into a handful of Adidas shoes; as shown below.
The first midsole plug - that has been inserted into the midsole of an Adidas shoe - was probably the removable plugs of the Adidas LA Trainer; apparently they altered the cushioning and performance of the shoe, though this 'claim' is debatable. The moulded midsole plugs are purely an aesthetic design feature, shown above, that are inserted to make the shoe 'look better' than to serve any performance purpose. The midsole plugs shown above are made from EVA; a polymer that has a soft 'rubber like' texture. While the potential exists that a moulded midsole plug is used to customise the cushioning of the shoe, Adidas have yet to describe one as such. Adidas shoes that have a midsole plug include: Adidas LA Trainer, Adidas NMD_R1, Adidas Cloudfoam 8TIS, Adidas NMD Runner PK, Adidas Originals Handball 5, and Adidas Originals Kegler Super.