This mod tries to prevent people from combat logging on a server, after a player is hit by another player, both of them has a configurable countdown assigned to them and if either of them leave before that count down is over they are killed and all of their items are dropped on the ground.
Currently the mod has 3 config options combatTime, allDamage, and deathMessage. combatTime is simply how long a player should be tagged for before they can leave, allDamge is weather you want players to get tagged only from other players or every kind of damage, and deathMessage is just the message displayed when someone leaves while in combat. The config file is located in config/combatlog-common.toml
You only need the mod on the server but it does not hurt to have it on your client as you would be able to use it on singleplayer worlds you open to lan.
Feel free to use in your modpack.