This thread offers no evidence that SpaceX plans (or had ever planned) to use mercury as an ion propellant. I think the "Starlink plans" part of the title is an unjustified editorialization.

Starlink thrusters use krypton,

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

edit: If you look at the text of the legal complaint, the only mention of SpaceX mercury is a *pure hypothetical*:

https://peer.org/wp-content/uploads/attachments/11_19_18_FCC...

- "If a corporation such as SpaceX, OneWeb, or another major space company were to deploy an entire proposed LEO constellation with cheaper and more powerful mercury-fueled thrusters on-board, they could be launching as much as 198 tonnes of mercury into orbit, which will then be fired off into the upper atmosphere."