Any registry that publishes an expiry date over RDAP or whois, which is most of them. Where a registry publishes none, the monitor says the expiry is unavailable rather than showing a failure.

Is this the same as certificate monitoring?

No. A certificate and a domain registration expire on unrelated schedules, and each has its own monitor type.

Monitoring

Domain expiry monitoring

Days until the registration lapses. The outage nobody has a runbook for.

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Free plan with 20 monitors. No card.

What it checks

Domain expiry monitoring

Seven days' notice is too late for a domain: a registrar transfer takes five to ten, and a lapsed domain takes the mail with it.

Some registries publish no expiry date at all, and .ro is one of them. Where that is the case PingFleet says so instead of reporting the domain down.

A daily RDAP lookup, falling back to whois, reading the registry's own expiry date.
Warnings on the days you choose, the same thresholds as certificates.
A clear "not registered" state, which is a different emergency from "expires soon".

A domain monitor in PingFleet showing 287 days until the registration lapses.

It comes with the rest of the platform

Every monitor type shares the same alerting, the same status pages and the same map: escalation with on-call hours, a public status page, checks from independent networks, and agents for anything the internet cannot reach.

Questions

Which TLDs work?: Any registry that publishes an expiry date over RDAP or whois, which is most of them. Where a registry publishes none, the monitor says the expiry is unavailable rather than showing a failure.
Is this the same as certificate monitoring?: No. A certificate and a domain registration expire on unrelated schedules, and each has its own monitor type.