I have high 2 digits of extensions in my VS Code, and yours is the only one that wouldn't have a verified publisher. And I certainly have more than one from solo developers.

Qt organization (because you mentioned it) also has verification. It displays a different message (because I haven't installed anything from them):

The extension Qt Core is published by Qt Group. This is the first extension you're installing from this publisher.

Qt Group has verified ownership of qt.io.

Visual Studio Code has no control over the behavior of third-party extensions, including how they manage your personal data. Proceed only if you trust the publisher.

EDIT: I'm sure there are other extensions that are also by unverified publishers. It was the first time I was hit with that message though.