https://news.ycombinator.com/item?id=42937148 (works)
https://news.ycombinator.com/item?id=42937147 (fails)
(Also via email.)
This suggests an item-count or time-based sliding window to me.
As I write this, earliest accessible content is <https://news.ycombinator.com/item?id=42937674> ... and that's just become inaccessible as I'm updating this comment.
I just checked against oldest accessible article (at this writing), and subtracted that from the newest comment (from the "comments" titlebar link):
43938990 - 42938990 = 1000000
Others can confirm this by going to the newest comment, and subtracting 1000001 from the contentID to find the first inaccessible comment.