What

• Skin::preloadExistence for footer links - Introduced in 2016 with change 314799 by me to reduce database queries on every page view.
• $wgFooterLinkCacheExpiry - Introduced in 2020 with change 582611 by @aaron, but currently unused (disabled by default and at WMF).

Context

In T299099#7630162, @Krinkle wrote:

[…]

When I last refactored this code, I considered proposing to remove this feature. Instead, I first optimised it so that it would no longer be a perf concern, and then forgot about it.

Removing it would mean the footer links behave (very slightly) differently. In particular, it means that they no longer get a red color if the target page doesn't exist for any reason. The downside of this would be that on a brand new wiki, it would be less obvious to the community that some of their default project pages have yet to be created. Another scenario is if a localisation update or interface admin changes the message that decides where the link goes to, and if the new target doesn't exist. It'd be nice to easily spot that through a red link. How often does someone with knowledge of how to fix it or report it, click on footer links to see if they still work?

On the other hand, we don't provide this affordance to sidebar links (although perhaps we should?) Either way, the sidebar has its own batch loader and cache, so it wouldn't be a performance issue there even if we did support red links there. Perhaps this would be a useful thing to surface [..] for admins, [..]

Proposal

Make the footer links always blue and without applying LinkRenderer features adding a class to the element for "stub threshold" and for "is redirect".

Rationale

• Contrary to my context comment above, the footer links are already always blue. When I observed the LinkRenderer database queries, I assumed these queries result in at least somewhat-useful behaviour such as making links red or blue. However the skin has, since the introduction of the footer links in ~2004, always called Linker->makeKnown and so the database metadata is never used for that purpose. It never gets class=new and thus never becomes red.

• The metadata feature for stub threshold was removed from MW core.

• The metadata feature for redirect status results in a class mw-redirect being added, which has no effect by default and I can't think of a even a theoretical use case where this is important to surface in the footer. At least redlink would be somewhat useful to surface to site admins (though not on every page view per-se).