An Insert Game, or Insert War, is the general term for a type of game in which players inserted user-made models in order to fight each other. Insert games usually consisted of an insert tool within the player's inventory and a simple baseplate on which to battle on. Games of this genre were generally criticized for the ease of abuse and spam within the game that one could do.
They have seen a decline in popularity due to the insert tool breaking over time from various Roblox updates.
Before the InsertService update released, insert games were commonly abused. Players had the ability to insert game-breaking models, such as a forever exploding bomb and admin scripts (which players could then use to loopkill players, kick/ban players, or even prevent new players from entering). Also, they could insert nukes to delete everything within the game. Prior to the InsertService update, certain models called "takeovers" manipulated the game to only allow the model owner to control the game, preventing others from using the main tool.
An update to the InsertService service has ultimately destroyed insert games as InsertService now only permits the insertion of official Roblox assets (such as hats) and models that the creator of the game owns. The pros and cons of this update varies: insert games are less likely to be abused, but are limited to using primarily gear as a result. Now, only bricks, clothes and hats can be inserted.