Hi Clem,
Not sure if you are in the same boat as me, I have multiple old Telstra email addresses not linked to accounts, Telstra reps on this forum have advised that they will stay, and Telstra by phone as well, but when I asked for it in writing I didn't hear back by phone or email. I expect unless they are tied to an existing paid account they will disappear. If you get any response please post it.