At Nextcloud we have all the source files in the relevant source repositories. That works well since that’s mostly SVGs for icons.
For mockups, we post the images themselves in the issue discussions, for example like in the spec for the Contacts menu. Of course the mockups always need explanatory text so that’s what the issues are great for.
Ideally people include a link to the source, for example hosted on their own Nextcloud. This is however not regulated. Usually modifications are done by either asking for the source, or modifying the image, or by just giving text feedback and discusing.
We also used to have a design repository with mockups and sources etc but it wasn’t really used and introduced an additional thing you needed to do for designers to get involved. Git and repositories are complicated enough. So we abandoned that concept. It also lacked the text context and discussion on the design we can have in the issues directly, and separated the design process too much.
Now for specific promotional material, and source files for logo, colors, print files, presentations, etc we have a specific promo repository. That works very well to have one public central place for all these materials.