The best alternatives to Slither.io are Agar.io, Airmash, and LaLoLi. If these 3 options don't work for you, we've listed a few more alternatives below.
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Koop.ai— Satisfy SOC compliance & insurance requirements in one tool
Occup4.fun is browser based game where you have to occupy as much squares as possible. Do this by clicking the square you want to capture. If you don't expand during 3 seconds, you lose!