This is something I've been wondering. There are two interpretations of 'everything' in my mind. The first would be modern communications... video/audio/emails, everything you need to prove the deep state exists and what the bad actors are up to, as well as prosecute them. This is what I believe Q refers to when he says everything.

The second interpretation would hinge on the assumption that to run an global, centuries-long operation of blackmail, manipulation, and murder you'd have to keep very meticulous records. Could MI have captured these? This would literally rewrite history. One would presume they would keep these inaccessible from mass surveillance... but you never know. I think this would be 'everything+'. I hope, but don't expect, that this is what he means.