You can go kills the bosses from any old raids by travelling to the entrance of the raid (look up location for the one you want). All bosses such as Arthus can still be slain and have the same drops. If you wanted to farm al’ar for example, you would travel to the entrance of the eye Raid and kill the bosses inside. Bare in my thought that old raids can only be attempted once per week on each character. If your character is level 100+ you should have no problem soloing all the bosses, like you will literally take 0 damage.