Einstein's initial clue that spacetime is curved

I believe the initial clue was the (accelerating) elevator. This gave the idea for gravitational lensing.

Light follows a geodesic, and as the elevator accelerates, photons entering perpendicularly to the elevator sidewalls, those photons' path will be curved. Only acceleration can do that, and due to the equivalence principle, it is the same as the effect of a gravitational field.

https://en.wikipedia.org/wiki/General_relativity

Thus, the gravitational field of a massive body must curve spacetime itself, creating a bent path around the body for light.

https://medium.com/starts-with-a-bang/this-is-why-einstein-knew-that-gravity-must-bend-light-2fafcc8b5532