Since Minecraft 1.17, new shulkers can spawn when one shulker hits another shulker with a shulker bullet, wich can be enonomy breaker in some servers.

This Spigot plugin disables this shulker breeding mechanic. Shulkers can still spawn during new chunck generation, or by placing spawn eggs, etc.
Installation
Just put the
PhoenixShulkerFix jar into your plugins folder. No further actions needed, it just works.
Commands & Permissions
This plugin has no commands or permissions.
Configuration
config.yml:
# Enable this, if you want to see every canceled shulker spawn attempt on the console.
debug: false