Sequential development methodology where requirements, design, implementation, integration, and verification proceed as discrete, signed-off phases. Long out of fashion for general software but never fully displaced in safety- and certification-driven embedded domains (aerospace, medical, automotive, rail) where regulators expect a linear paper trail.
Treat waterfall as the right tool for safety-certified, contract-driven, or hardware-coupled programs where a paper trail is mandatory — and use iterative methods elsewhere. AI tooling now dramatically lowers the cost of producing the per-phase artifacts (specs, traceability matrices, test plans) that historically made waterfall painful.
Re-classified Stable (from Declining) for 2026: AI assistants that can consume and produce specifications, traceability matrices, and test plans are quietly making waterfall economically viable again for teams that need its compliance properties. Worth re-evaluating if your team abandoned it years ago purely because the documentation cost was crushing.