Can triggers or automations be used with the condition of unsharing a ticket with another Support account?
No, there is not currently a condition within business rules that accounts for the event of unsharing a ticket.
A workaround to this would be to utilize macros that apply a tag when an agent unshares a ticket. Doing so, would allow the agent to create a trigger or automation that would run whenever a ticket is updated with your unsharing tag. For information on macros, see Using macros to update tickets.