This alone likely wouldn't convince flat-earthers to abandon their theory, but it could definitely shed some doubt and, together with other means, contribute to the diminishing of the belief.
Foucault pendulum is a device made to prove the rotation (around its axis) of the Eart. It seems that flat-earthers state that the Earth is not only flat but that it doesn't rotate or move in any way. They also often point to the lack of direct empirical (sensory) evidence confirming that the earth is round as a reason to believe that it is flat. The pendulum provides that evidence.
A heavy enough weight is suspended on a long enough wire (a height of a church ceiling or similar). The pendulum is pushed and as it swings, during the day its swinging plane appears to be rotating - the pendulum could draw patterns in the sand, push off objects placed along the edge of the circle which surrounds it, etc. This apparent side movement of the pendulum is caused by the Earth's rotation.
It was first built and publicly exhibited in 1851, to prove the phenomenon to the general public. Then there were no possibilities to view or photograph the Earth from high above, so people were closer (evidence-wise) to flat Earthers than we are now.
There are plenty of those pendulums exhibited around the world in science museums, observatories, etc. So a flat-earther could simply go to the closest place which has one, to observe the phenomenon with their own eyes. Even better - if there's distrust in others, with some effort one could build such a pendulum him/herself, it's way easier than going to space with a homemade rocket.
An issue with the Foucault pendulum phenomenon is that the more you get into the details of how it works, the more unclear it gets (from personal experience). It's the easiest to apply the most intuitive concept - the Earth spins beneath the swinging pendulum, while the pendulum stays fixed to its swinging plane, which is not exactly correct but is enough to get someone interested. Leaving the theory aside, the view itself should be pretty convincing.
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