I don't believe that any such shortcut exists. Probably not many people need it anyway, as most topics peter out reasonably quickly, so as long as one doesn't join one of the nonsensical counting threads in Off-Topic, the number of alerts should be reasonable. I am following probably a couple thousands of topics, and most of them went inactive ages ago.
Unfollowing Topics
If you need help, please contact our Help and Support team.
Is there a shortcut to doing this rather than one at a time? tyia