GitLab seem to accept jobs containing anything, including keywords that aren't normally allowed in a job. I've not tested, but this question even suggest that one could even make the hidden job a scalar. Or for example:

..fubar:
    foo: bar

of course I don't expect it to be usable with the extends keyword, but it seem to be possible to use with the !reference tag (which is what I'm looking for).

The documentation isn't very clear on whether it is supported. The documentation says fx:

To temporarily disable a job without deleting it from the configuration file, add a period (.) to the start of the job name. Hidden jobs do not need to contain the script or trigger keywords, but must contain valid YAML configuration.

Hidden jobs are not processed by GitLab CI/CD, but they can be used as templates for reusable configuration with:

Is this to be interpreted as a hidden job is only required to be valid YAML (ie without regards to what the documentation otherwise says about the content of a job), as long as it isn't used in a way that would produce an invalid non-hidden job (or other key) by inserting contents from it using fx extends, !reference or YAML alias