1.8 support is a big step from 1.9. Here is a checklist of things that need to be considered for this update, on top of the normal things that are needed for many updates:

Combat

• Reimplement sword blocking.
• Remove combat cooldown timers.
• Block the offhand slot, and the swap hands keybind.
• Fix potion items.
• Check knockback works as expected.
• Make sure players can't push other players.
• Remove stat.pickup.* and stat.drop.* from menu and commands.
• Revert blaze powder from brewing stand container.
• Implement changes in projectile motion.

Not combat

• Fix entity riding (passengers -> riding). Lots of protocol-related things here as well as making sure things work.
• Reimplement 1.8 boats.
• Revert parkour mechanics to 1.8.
• Revert trapdoor climbing.
• Remove 1.9 options from the command block GUI.
• Don't create grass paths from right clicking grass with a shovel.
• Revert placement behaviour of banners, signs, doors and lily pads.
• Check tool mining speeds.
• Prevent buckets from being filled from cauldrons.
• Use cross layout for iron bars and glass panes when there are no connecting blocks.
• Check whether ender pearls can be thrown in creative.
• Remove 1.9 achievements.
• Reverse addition of scoreboard objective criteria.

Problems encountered while working on this

• Sword blocking transform is slightly different compared to real 1.8.
• Item uses have repeated cooldowns when the item can't be used (try holding right click with a food item when you can't eat)
• Trapdoor climbing is still different due to the ladder hitbox

interesting