respawn radius

I dont reallt know how to title it... Basically, for a modified anarchy server im adminning for school, i need to be able to stop people form going out far. I thought a world border was too harsh since people like the exploration factor of the game. I landed on the idea of limiting a radius (say between XY +/- 2000) In which a player can set a respawn point, and they will be blocked from doing so outside that area (ideally warned they left as well). Ive asked various chat bots and scoured the internet to no avail (AI keeps referencing some SpawnX NBT that doesnt exist), any ideas?