First, check deliverability before blaming subject lines: a sudden cliff usually means messages stopped reaching inboxes — new blacklist listing, an authentication record broken by a DNS change, or a provider reputation drop after a bad send.
Second, remember opens are partly fiction now: privacy features fire phantom opens and block tracking pixels unevenly, so measurement shifts can masquerade as performance shifts, especially after list composition changes.
The diagnostic sequence: run a deliverability check on the sending domain, review the last few sends for bounce spikes, test-send to your own accounts at Gmail/Outlook and see which folder it lands in. Only after those pass is it a content problem.