This is incorrect (or at least only technically correct). This is only true for subdomains with public, trusted CA signed certificates since certificate transparency has existed and only for subdomains with a specific, non wildcard certificate.

https://crt.sh can find your subdomain only when it doesn't have a wildcard certificate(*.somedomain.com)

Thanks for mentioning. I checked it out, and am learning lots of new stuff (ie, realize how much I do not know).

Doesn’t find any of my semi secret subdomains.