Gold is lethal to the Cybermen, which is why they tried to destroy Voga, the planet of gold in "Revenge of the Cybermen" and why the Fifth Doctor (Peter Davison) used Adric's gold badge for mathematical excellence to kill a Cyberman in "Earthshock".

Source: https://screenrant.com/doctor-who-season-14-rtd-clues-meaning-tigella-kingdom-gold/

The episode ends with the visual of a single tear falling from the empty eye socket of Bill’s Cyberman form — a chilling reminder that each and every one of the Cybermen were once human beings.

Source: https://gamerant.com/doctor-who-episode-saved-cybermen/