MediaWiki:Pagecategories was created by Midom in 2008 with the edit summary "the rare occurrence of singular 'category' adds few milliseconds to page load :)" (this was deleted a few months later, only to be recreated by Masem in 2009). This was also communicated in the Signpost at the time:

There were other related miscellaneous changes such as changing MediaWiki:Pagecategories from the default {{PLURAL:$1|Category|Categories}} to just Categories; on a wiki as large as the English Wikipedia, and for a message that needs to appear on most pages, this actually makes a significant difference.

Whether this micro-optimization was still needed was questioned in 2016 on the talk page, to which @Nikerabbit replied with "[m]y guess is that this is less of an issue nowadays, but I am not able to say for sure."

It has been six years since the talk page discussion, and 14 years since the original change; in that time, PHP, MediaWiki, and the WMF server infrastructure have changed a lot. Is this optimization still needed? Even if it has the same absolute impact now (which I doubt) as it did in 2008 (a "few milliseconds"), is that still as significant a concern? Or can this page be deleted now, restoring the default message? Should I simply BOLDly delete it myself and see what the fallout is? (: