What makes you think they don't hold that data anymore?
But "old content" is maybe not so valuable for engagement-based revenue.
If Reddit wanted to restore the deleted stuff, I suspect that they still have copies of it.
If user sentiment was the barrier to restoring the deleted (say, in non-"sensitive" subs), then waiting until after IPO seems a good idea, so maybe we'll see that now.
I always read to edit a comment before deleting it. To actually delete it. Make the data in the DB worthless, then delete. So even if the comment is restored, it’s nonsense.
Most of the comment delete scripts work like this.
Which is one of the reasons that I doubt that's a barrier to restoring the comments and posts, if they wanted to.
People should think of the delete tools as being a symbolic protest, with the collateral damage of denying random nice people the comments/posts later -- but not as providing protection from Reddit reanimating their comments/posts later, nor denying it to someone who really wants to dig it up.