Are old nodes automatically archived over time?
Drupal does not archive or otherwise delete or modify content automatically over time. There are a few possibilities as to what happened to node1 including:


An administrator or someone with permission to delete nodes of that content type deleted it
there is a rule or other module configuration that deletes nodes after a certain amount of time
Node 1 was deleted a long time ago and someone changed the front page after the fact (not likely as I think drupal verifies the path when you save that setting)


Drupal does keep a record of when nodes are deleted in the watchdog table. The watchdog table has a UI which can be found at /admin/reports/dblog. You can filter this list by type, but note that Drupal only stores a finite number of rows in this table with First In First Out as a rule. So it's possible that the record of node 1 being deleted has also been deleted.