sky block border

What have you tried so far? ( Don't tell me "nothing" i'll won't spoonfeed you )

 

gubhub? is that a fish sort?
Ok wasn't funny
Just by looking into the github project of ASkyBlock doesn't give you a view of how to create the visual border.
You can create a plugin using the ASkyBlock inbound API that gives you the island location of players ( Class: ASkyBlockAPI )
Now you have the center of the island and you can use a radius or field that should be used as border. Now its your choice to send worldborder packets to the client so he just can't get out of it or you use something similar ( not easier ) and grab the locations of your field and you check something like if the player is in the border or not.