EFFECT any player under a specific height anywhere in world?

Solution 1:

as doesn't shift the execution position, so you're still executing from the command block's position. You just need to add at @s:

execute as @a at @s run effect give @s[y=0,dy=57] minecraft:glowing 3 1

Also, in theory this should work, without execute:

effect give @a[x=-30000000,dx=60000000,y=0,dy=57,z=-30000000,dz=60000000] glowing 3 1