I was searching for some Gradle documentation in the user manual and noticed that the search feature on the documentation site seems to only work when the latest version of Gradle is selected? I checked that my browser wasn’t blocking any cookies, but the site seemed to behave the same in other browsers as well.
Yes, that’s correct. The search box even used to not be displayed on any pages but with /current/ in the URL. The box shown now is also the old one, not the one you see on current. I assume that it simply is an error that with the new search the old search box now shows up for non-current pages and already mentioned this at Search functionality in gradle docs is not as good as it previously was · Issue #29451 · gradle/gradle · GitHub
I appreciate the confirmation; I’m glad I wasn’t doing something wrong on my end! 
It would sure be nice if they remedied that (and soon) given not everyone is using the latest Gradle version.
Well, the fix would most probably be to again hide the search box on anything but /current/ pages like it was ever since, so …
So, if I understand you correctly, their ideal solution would be to not allow searches on older Gradle version documentation?
That’s hardly a good solution since, again, not everyone is using the current version of Gradle.
What I say is, that it always was like that.
You never got any search box shown on any page that did not have /current/ in the URL.
That the search boxes are shown there just happened recently and most probably is a bug due to switching to a new search implementation.
Fair enough, but my point is that all versions of the documentation should be searchable via the same mechanism, not just the current version. If that’s not been the case in the past then I question who’s making the decision not to provide that functionality on the site.
And I fully agree as you can see from my comment on the mentioned issue.
Actually I was wrong, at some point all pages were searchable, but it was removed.
As to who decides it, well the Gradle folks of course. For example here: User manual search does not work for older versions · Issue #27769 · gradle/gradle · GitHub
Is there anyone at Gradle whose attention we can capture to have this issue fixed, perhaps? 
This is mainly a community forum where users like me help other users.
If you want definitive attention by Gradle folks, you need to open an issue, or post to the issue I mentioned.
But I lightly doubt creating a duplicate of the issue I mentioned will have more success. 