ar5iv tracks the arXiv collection with a one month lag. Exactly as to signal that this is not the "official" arXiv rendering. It is also a showcase predating the arXiv /html/ route, but largely using the same technology. Nowadays maintained by the same people (hi!)
There used to be another showcase, called arxiv-vanity. They captured what happened pretty well with their farewell post on their homepage:
> View any arXiv article URL [in HTML] by changing the X to a 5
The line
> Sources upto the end of November 2025.
sounds to me like this is indeed intended for older articles.