Bitcoin Core publishes a new major version about every six months and maintains the latest three. When a version falls out of that window it reaches end of life: the project no longer generally back-ports fixes to it, security fixes included, and its operators are expected to upgrade.
Running one is not a consensus problem in itself. An end-of-life node still validates blocks by the rules it knows and follows the same chain; after a soft fork it accepts the new blocks without enforcing the new rule itself. Its exposure is to vulnerabilities disclosed after its window closed, and the project's security advisories say which versions each one affects.
Counting end-of-life nodes means joining crawler user agents to the life-cycle table, with the caveat that every version string is self-reported.