(Reading this as “while un-tagging others, even if they are more recent”.)
Then it’s not about news but rather about featured / highlighted projects. News and blogs are ordered by publication date, and there is an archive, and stuff does not vanish from the archive (so, no un-tagging). In the case of news, the active projects would naturally generate more news, which would then displace the outdated news of no-longer-active projects from the slider.
But now what if you want not a project news feed but manually featured projects or articles. In that case, you’d only want to give some people control over what content gets featured. I’d propose a YAML-in-Discourse-post block with the configuration section of the webkit slider element. It would be editable by the author, moderators and admins, and would reference the topic IDs of featured content, for example by including the full links. Changing a link in a YAML section is not asking too much. (And if somebody would create malformed YAML that way, the webkit could look into the older post revisions of the YAML-in-Discourse post to find the last working version, and use that. No big issue.)
Afterthought, with a better proposal: Since the webkit has this GUI for creating the JSON config file that defines the website, topic IDs or links to include into a slider would be entered via this GUI. This requires a way to comfortably deploy changes to that config file, which might not be available in early releases. But for the medium term, that kind of solution would be a natural choice.