Just had a GM accidentally delete a thread when they went to edit it.

Given that the buttons are right next to each other, I think an interstitial page would be appropriate for the irreversable one.  Or a checkbox next to the link to enable it "Delete Thread [] _{check to enable deletion}", or something.  Whatever's easiest to implement really.

Thank you.

Best as I can see, there's already a secondary 'Confirm you wish to delete the thread <thread name>' button that comes up after you've clicked the initial 'Delete Thread', and has to be clicked on both Responsive and the old site in order to actually delete the thread. For PMs, the Purge Thread button also has a secondary confirmation that works the same way, on both old and new RPoL.