Yes, some of the historical PDF releases (particular the 4e titles) were available for download again as soon as the DNDClassics site went live, provided that you'd bought them previously. Others were only added later (often a day or two before they were made generally available again), or not at all (all of the OD&D titles).
By my count, 38% of all D&D releases are currently available again as PDFs from DNDClassics. At the current release rate (roughly five items per week), the entire D&D back catalog should be available by May 2017.
Here's a graph of what proportion of titles from each edition is currently available as PDFs:
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