If you use TLS certificates for your services, and ACME (Let's Encrypt or alternatives), all the domains for which a certificate was emitted can easily be queried.
You can visit the following website, type a domain name, and you will immediately have a list of existing domain names.
crt.sh Certificate Search
In such situation, if you planned to keep a domain hidden by not sharing it with anyone, you got it wrong.

Source : https://dataswamp.org/~solene/2024-09-10-self-hosting-at-home-privacy-issues.html#_TLS_certificates_using_ACME