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When I designed it was intended to be just a temporary solution until I had money for a "proper" BLTouch. So I designed around things I have at hand. The result was so good that I just gave up buying the BLTouch.
In general, electromagnets are heavier than servos (for the same force) and need more current and also would complicate things - I would need to design one that not everybody would be able to build or find a specific off the shelf one that not everybody would be able to find. A servo is readily available and even the poor quality ones will handle the job.
But.... BLtouch does use electromagnets to deploy and retract the probe as the force needed for this is very small so it is a possible solution. I would put it below the flag and add a small permanent magnet to react to the electromagnet. The main issue is that I really do not know how to design a proper electromagnet
I am designing a new BFPTouch that uses a nano servo and a pair of small magnets as spring but it still at the concept level :D As this one is working (still the same servo and sensor from the first prototype!) the new one is not a high priority project. :D