Are there any reliable anti-exploit systems that hold up in 2021?

Make serversided anti exploits using magnitude, other checks…

But that will effect people that is lagging

No It will not, if you make it correctly - there will be no false positives for laggy players.

you said mine was false… but it wasn’t. He was asking if you could see if a player changed their walkspeed… not detect how far he’s moving every so often. Exploiters can change the index by getting a raw metatable of the game and change values in there, such as an index and it’s property values.

No he did not he said “but I just need a general anti-exploit system that prevents basic exploits like walkspeed, teleport, noclip, fly, etc.” he did not say when it gets changed and any way that’s how you detect speed using server.