Hi Loiroi,
Most foam mattresses don’t have or even really need side support if they have a support core that is firm enough and the comfort layers aren’t too thick and/or soft for the person sleeping on it. Having said that … there are exceptions to every rule and there is more information about edge support and foam mattresses in post #3 here and in post #33 here. With a foam mattress that is a suitable firmness level relative to the person sleeping on it (both in the support core and the comfort layers) then it normally wouldn’t be a significant issue for most people unless you often sleep or sit with most of your weight concentrated on the very outside edge of the mattress although the outside edges of a foam mattress would generally be softer than an innerspring mattress that has a specific edge support system.
Phoenix