From 20w13a to 20w17a, striders could get the No Gravity tag set to 1. This allows the strider to fly. The player can ride this flying strider but on a server, it'll continuously kick the player for trying to fly. The NoGravity tag persists after the update.

Can Striders fly in the overworld?

Unlike piglins and hoglins, striders can exist safely in the Overworld, as long as they are kept away from water and rain.

Why do Striders fly?

For instance, if the strider is living in small wetland and temperatures are rising, the habitat is likely to disappear. Thus a mechanism is triggered so the next generation of water striders has wings, allowing them to fly away from their drying wetland.