If you've got super-secret-subdomain.example.com, a wildcard for *.example.com doesn't expose its existence. A non-wildcard certificate does.
There's a bit of security-via-obscurity going on here, but sometimes the need for such a thing is out of one's control.
Exposing existence is a separate matter, the only thing I can think of is that you're referring to those TLS certificates with multiple domains (almost just as bad as a wildcard certificate). Now that TLS certificates are free, each subdomain can indeed have its own certificate, thereby not causing any exposure.
Lets say you have super secret domain xsrihghufgyssw.foo.com, it is published in your DNS and it is publically reachable, but no one ever knows to look up that domain to reach the website. It is almost impossible for someone to randomly stumble upon or guess that domain name. With certificate transparency there is a log of that certificate being registered so if some group like DDOS for hire wanted to take your services out, they could use all those names as targets.
The TL;DR version: Every certificate goes into the log. *.example.com going into the log is no problem for hiding a particular subdomain.