Hey hey!
Mojang has released a custom snapshot to test new combat mechanics.
You can read everything about that test snapshot here:
https://www.reddit.com/r/Minec…pshot_to_test_new_combat/
We've grabbed the server files and made them available for you here:
https://aternos.org/software/v/snapshot/combat
Please only use them on new worlds and to test things and don't update your current world to that snapshot.
(That's why we require you to reinstall your server to install the snapshot)
Happy testing and don't forget to give Mojang feedback on reddit!