(Sophivorus asked me directly elsewhere.)
@Sophivorus That sounds fine to me.
The use case for these namespaces (Gadgets 2.0) has not gone away, and is still actively being worked on. But, the latest version of MediaWiki no longer requires custom namespaces to implement them, we have content models instead. Which means if/when Gadgets 2.0 gets completed and deployed to mediawiki.org (which I hope it does!), it will not clash with these documentation pages.
In February earlier this year, I have updated the Gadgets extension to completely remove these namespace definitions, thus making it clear that is no longer reserved in any way.
More details in T31272#9574679, and T31272#9574686, and the linked code changes on Gerrit.