I believe that nextpnr has integrated both a simulated annealing placer and an analytical placer (based on this publication from 2019: https://arxiv.org/abs/1903.10407) (see here for the work the analytical placer is based on: https://ieeexplore.ieee.org/abstract/document/6339278). I think they are also working on electrostatic placement but I'm not sure what the current status of that is.