This is what they say the 'deep things' are these days - W 07 11/1
We should not think that “deep things” are necessarily difficult things. Knowledge of “the deep things of God” is obscured from most people...
Examples of Deep Things
Jesus is the primary part of the “seed” of God’s heavenly “woman” referred to at Genesis 3:15....12 A second example of deep things that have been revealed to us concerns God’s purpose to select 144,000 from among mankind. (Revelation 14:1, 4) We accept the teaching that all of the anointed ones living on earth at any given time constitute “the faithful and discreet slave” that Jesus said would provide timely “food” for his domestics.
Even the 'deep things' are subject to change.