The snapshotting process is still manual and we don’t have a specific deadline for that. I would not suggest using that as the reference for what distros are active.
Instead you can directly query rosdistro for that in closed form from the rosdistro index that’s actively maintained and is the explicit source of truth of what’s active. eg Iron going active and Galactic going EOL
#!/usr/bin/env python
import rosdistro
r = rosdistro.get_index(rosdistro.get_index_url())
active_distros = [k for k, v in r.distributions.items() if v['distribution_status'] == 'active']
print("Active distros: %s" % active_distros)
@tfoote is spot on here. In addition to declared state in the rosdistro index, the state of rosdistro during each distribution sync is tagged by the ROS boss during the sync and pushed to GitHub so you can also see when a specific distribution was last updated.
To answer the specific question regarding snapshots for Iron. Sync snapshots for non-LTS distributions are not published, so Iron will not appear on the snapshots site until the end of its support lifecycle and the final snapshot is published.