So why was that major thread deleted?

You’re going to have to be more specific.

"Do you guys know what happened with that big thing at some time in the past? You know, the thing?"

Fam, you need to be more specific. Also, the mods aren't normally going to tell you if it's not your thread.

However, given that 80% of the recent posts are about the Pax shit, I'm going to assume it's one related to that. In which case, it was removed due to it having/being personal attacks (Saying to fire DZK or other Rioters, verbally attacking boards users, Rioters, or other people), spam (Including low-content threads or threads that are nearly-identical to other threads posted by other users), or it being politically-charged (Including many topics that are not necessarily political, but are an otherwise divisive or argument-inducing topic, such as sexual discrimination and other issues). All of those kinds of discussions are forbidden, so the mods either deleted it, locked it, or edited it.