Breaking

node >= 12 is required. Time to upgrade that old-ass server you never touch.
peerDependencies are now excluded by default. Peer dependencies should use the lowest possible version that works. The old behavior encouraged a bad practice of uprading peer dependencies. You can use --dep prod,dev,bundle,optional,peer for the old behavior (#951).
Dependencies with > will be converted to >=. The old behavior was causing upgrades to > [latest] which was impossible (#957).

Other

Typescript! There is a new build process, so if you have any issues with the executable or types, please report. It should be a non-breaking change if I did it correctly (#888).
When using npm-check-updates as a module, vm (versionmanager) is no longer exported. It was previously exposed for testing purposes, but was never part of the official API.