This plugin that I made over the weekend allows administrators and server owners to prevent players from using their Elytra whilst in combat. 
Important note: OPs by default have the permission nodes to bypass the checks, if you're wondering why it might not be working, try deoping yourself.
Config stuff:
Several values can be customised under NoElytraDuringCombat/settings.yml:
combatTimeInSeconds: How long the combat timer lasts in seconds.
notifyInterval: How long in seconds between each denial message.
Messages can also be customised under NoElytraDuringCombat/lang.yml
Permission nodes:
nedc.bypass: Bypass all checks for testing purposes.
nedc.save: Save all data manually via /nedcsave.
nedc.reload: Reload the plugin.
Source code if you need a peek: https://github.com/Cvahotni/NoElytraDuringCombat
Important note: OPs by default have the permission nodes to bypass the checks, if you're wondering why it might not be working, try deoping yourself.
Config stuff:
Several values can be customised under NoElytraDuringCombat/settings.yml:
combatTimeInSeconds: How long the combat timer lasts in seconds.
notifyInterval: How long in seconds between each denial message.
Messages can also be customised under NoElytraDuringCombat/lang.yml
Permission nodes:
nedc.bypass: Bypass all checks for testing purposes.
nedc.save: Save all data manually via /nedcsave.
nedc.reload: Reload the plugin.
Source code if you need a peek: https://github.com/Cvahotni/NoElytraDuringCombat
