For my personal domain: it got the ones I have on the SSL cert alternative subject names, made up three, returned one I deleted more than a year ago, and didn't find two. Very curious.
Those SAN and CN names will appear in publicly visible certificate transparency lists ( https://en.wikipedia.org/wiki/Certificate_Transparency ): so if you ever get a TLS certificate for a super-seeekret internal sub-sub-sub-domain-name from a major CA then it won't be secret for long. The only way to keep a publicly-resolvable DNS subdomain confidential is to either get a wildcard cert for the parent domain or find a dodgy (yet somehow widely-trusted) CA that doesn't particiate in CT - or use a self-signed cert.
This subdomain.center database returned one of my "private" sub-sub-domains (which just points to my NAS) for which I did get a cert from LetsEncrypt, but it doesn't have any of my other sub-sub domains listed (despite resolving to the same A IPv4 address as the listed subdomain) because those subdomains have only ever been secured by a wildcard cert.
