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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Notion— Get over $6,000 worth of Notion + unlimited AI, for free
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!