For the sake of documentation, I introduce here a proof I found today (11/13/2019), A note on “Measuring propagation speed of Coulomb fields” by R. de Sangro, G. Finocchiaro, P. Patteri, M. Piccolo, G. Pizzella by R. de Sangro, G. Finocchiaro, P. Patteri, M. Piccolo, G. Pizzella, published in 2016.
Basically they followed Feynman's suggestion by deriving the electrical field from the Heaviside-Feynman formula and comparing the result to standard result obtained from the Lienerd-Wiechert potentials found in many textbooks.