The rider should follow the rules of the mount getting knocked prone
There isn’t anything in the core rules that covers this exact situation, but a common-sense approach should be enough to make a ruling.
The Find Steed spell says:
When the steed drops to 0 hit points, it disappears, leaving behind no physical form.
The rules on mounting and dismounting state:
If your mount is knocked prone, you can use your reaction to dismount it as it falls and land on your feet. Otherwise, you are dismounted and fall prone in a space within 5 feet it.
If you combine the two, and consider that instead of going prone the mount has simply disappeared, allowing the rider to make the reaction dismount seems reasonable. If they chose not to use their reaction, then they fall prone.
A reasonable DM may decide to just call for a Dexterity saving throw from the rider as well, landing on their feet on a success, or falling prone in the dirt on a failure. But I think the rules lean more towards the dismount using a reaction. The rules on mounting and dismounting mention this:
If an effect moves your mount against its will while you’re on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it.
But in this case, the situation is not the same. The save is to fall off or stay on the mount, which clearly isn’t an option in this case where the mount doesn’t exist any more.