Actually I'm thinking of a different method that doesn't use shields. Since I don't know how to code server plugins, I can't really test my theory myself.I personally never liked blockhitting, and it just felt too weird of a mechanic attached to a sword, so I dunno if I would want it back.
To answer your question: Yes, it's possible. The method behind it is basically using a temporary shield in your off-hand whenever you right-click with a sword, but that shield only mitigates 50% rather than 100%. Speaking objectively though: bringing the mechanic back would hinder the use of off-hand + sword combos, which a lot of people have grown accustomed to and would actually prefer.
Also move slower / momentum change when right clicking which is a lot harder to do right.Actually I'm thinking of a different method that doesn't use shields. Since I don't know how to code server plugins, I can't really test my theory myself.
You can easily test to see if someone is right clicking with their sword in their hand. After that apply an X% to their total damage reduction. Jugg has a built in damage reduction modifier so I'm pretty sure this could be done whenever someone right clicks with their sword. The only thing you would be missing out on is a visual notification that you are indeed blocking.
Walk speed can be changed on the fly without having to use potion effects, but it could cause a few issues for sure.Also move slower / momentum change when right clicking which is a lot harder to do right.