Plugin couldn't load to test out, most likely because the plugin is compiled incorrectly.
As per the error on load:
""
org.bukkit.plugin.InvalidPluginException: Cannot find main class `com.yourname.killeffects.App'
""

When decompiling the plugin, that class doesn't even exist.
The two classes that DO exist:
- KillEffects
- PlayerKillListener

Ok, so looking at the source code, theres a couple oddities here:
1) The need for Geyser is confusing, as the code does absolutely nothing with Geyser.
On enable, the plugin checks if geyser is enabled, if true, it sets a field to true, which has a method to grab from the field, but is never actually used anywhere.
If false, the plugin shuts down.
The plugin does absolutely nothing with Geyser.

2) The one listener class has one listener only.
PlayerDeathEvent... and all it does is adds some hardcoded (ie: not configurable) potion effects to the killer and victim.
Funny part is I dont even think you can add potion effects to a player in a death event, cause they... died. But regardless, theres really not much here.

Summary:
- Plugin wont load due to incorrect plugin.yml and incorrect compilation process.
- Even if it was compiled correctly and could actually run... theres really nothing exciting in this plugin.
- The plugin is extremely basic, gives AI vibes and/or beginner (as in, just started learning java 2 seconds before making this plugin) vibes.