A lifecycle deadline is the point at which a product reaches the end of its supported update period. After that date, the vendor may stop issuing security patches, bug fixes, or technical assistance, even if the software still works.
In cyber security, lifecycle deadlines matter because unpatched software becomes harder to defend over time. Attackers often look for systems that are past support, since known flaws may remain open indefinitely. Defenders use these dates to plan upgrades, retire old services, and add compensating controls such as segmentation, hardening, tighter access, and monitoring while migration is underway. For systems like email servers, identity tools, or remote access platforms, missing a lifecycle deadline can turn routine maintenance into a major risk-management problem.