I made a horse race track and at the start everyone gets slowness. At the end when they cross the finish line I want them to lose this effect. How can I achieve this?
The command for getting rid of all effects is
/effect *player* clear, so if you put a pressure plate or something on the finish line connected to a command block, the block command would be
effect @p clear. If however your racers have other effects that you don't want to remove, use
effect @p 2 0, which will remove slowness only.