Stacks is a design system that is developed for Stack Exchange and maintained by Stack Exchange developers.
In this way, it is both, about the Stack Exchange network and third-party.
When is a question about Stacks on-topic here on Meta.SE?
As the design lead on Stacks, I agree with @MEEisJohannGambolputty’s answer. I’m happy to discuss Stacks just about anywhere on Stack Exchange and its meta communities, especially if we’re introducing bugs :)
Stacks powers each new feature at Stack Overflow, and will get used more as we continue to refactor old views, so it’s totally appropriate to discuss it on meta.
However, I’d like to speak to the corollary. You should NOT be filing feature requests as new issues on the Stacks repo.
If you’ve discovered a component doesn’t work in IE11, that would be welcomed as an issue on our repository. For example, we had a contrast issue with our radio button component.
Edit: I forgot to mention that Stacks is fully open source. Pull requests are absolutely welcome! These could include bugfixes, typo changes, or any other works in progress.
In my opinion, a question is on-topic here on Meta Stack Exchange, when it is about the application of the design system in the software that powers the Stack Exchange network.
This means a question about a bug in the design system is on-topic when this bug occurs in the Stack Exchange software. A feature-request is on-topic when it has something to do with the application of Stacks here on the site (for example a request for a certain component to use Stacks).
I would furthermore recommend the following rule of thumb to determine whether a given question is on- or off-topic:
Was this question triggered when you used Stack Exchange or when you read through the Stacks documentation?
If your question fails this test, it is probably better asked on the GitHub issues page.