I know it is easy. In this particular example, it is also trivial to write it from scratch.

But, if UIOP:STRCAT is such a "standardized" solution as you claim, then I believe it should become part of a (hypothetical updated) Standard, not a second-class citizen behind a member of the ancient aristocracy like Lord CONCATENATE.

What I mean is that I think there should be some kind of mechanism so that standardized solutions emerge (from widely used libraries for example) and become part of the Standard. Like it happily happens in many other domains.