npm 7 runs on Node.js 18 (npm 7 requires Node ≥ 10.13.0). However, npm 7 introduced strict peer dependency enforcement by default — a breaking change from npm 6. Projects with unresolved peer dependency conflicts will fail to install. This is a project-level issue, not an engine incompatibility, but it commonly causes install failures when downgrading from npm 8/9 to npm 7.
--legacy-peer-deps