Thanks for the reply. I've already checked the Meta and the Wikitech page months before, so I understood it was ridiculously difficult to delete a thread via the previous version of Mailman. The reason of me asking this question now is because the Mailman 3 update, which let me be a bit hopeful about whether it'd become easier (or practically possible) to delete a thread.
Now I know how the "delete" circumstance be, is it possible to only edit out the sensitive content of the thread instead of completely wiping everything off? If possible, is it less disruptive? It will decide my choice later on.
P.S. Though this list is a "List for discussions and announcements related to lists.wikimedia.org", and I knew listadmins-announce (so I choose to post here), but I still offer my sincere apology for bothering Andre and everyone else on listadmins@lists.wikimedia.org, as I don't know where else to ask the question ><.
Thank you, and sorry.