Mega threads are just threads with several thousand replies. Since you need to read several thousand posts, they’re ideal for this purpose so you can just leave the computer on and don’t have to actually sit and scroll through the inhumane requirement for level 3.
I linked this before but I’ll link it again:
There’s also the Mercy mega thread, but this thread has gotten so big that the forum tends to bug out when scrolling through and you occasionally need to refresh and start from where you left off:
Then again, that thread has over 20,000 replies. The bastion thread on 5000 is more than enough.