Ability to disable the swing animation in newer versions to make combat feel like it did pre 1.9 but only visually. There is a good mod out there called Old Swing, you can look it up. It also has other eye candy features which are nice. Also within the mod, it allows you also to bring back other older features like the old button hover with yellow text with how it was before 1.15, this is very useful for some buttons as it makes the text more visually apparent. Along with, old item holding/switching, sneaking, etc.
Another mod besides it that would be nice is Sword Blocking. It only works if you have a shield and is only visually client side. It makes the shield dissapear and shows the old sword blocking animations.