We did have one before......

perhaps we could also use a component for core?

There is blocker bug 384 instead of a keyword which makes this a WORKS FOR ME if there are no objections. Arguments for making this a component welcome...

(In reply to comment #3)

There is blocker bug 384 instead of a keyword which makes this a WORKS FOR ME
if there are no objections. Arguments for making this a component welcome...

[[mw:Developers/Maintainers]] lists some maintainers for this "component" which is not actually a component, so if they find it useful for default cc's and clarity to bugzilla users it seems a legit request.
It's also one of our biggest/most cluttered tracking bugs.

(In reply to comment #4)

so if they find it useful for default cc's and clarity

Then somebody should ask them. :)

As https://bugzilla.wikimedia.org/show_activity.cgi?id=27283 shows there WAS a keyword for "postgresql" at least until 2012-08-04, and probably some Bugzilla admin removed it in favor of the blocker bug (and probably that someone did not document the changes).
I'd go for anything that's best for developers interested in postgresql support.

Tracking bug is okay, although making people memorize tracking bug numbers and/or search for them is not so nice.

Tracking bug has the one advantage that you can CC on it and get bugmail on changes of the dependency list, while that's not possible for keywords. Disadvantage is discoverability.