OTRS and RT allow you to watch a ticket. Thus you can chose to be notified for changes to a ticket (opt-in). I know of no particular opt-out feature, though at

OTRS and RT allow you to watch a ticket. Thus you can chose to be notified for changes to a ticket (opt-in). I know of no particular opt-out feature, though at least for OTRS it would be rather easy to write an add-on that makes you watch all new tickets in a particular queue, of which you could opt-out by stopping to watch a ticket.