- Too old, no software can decrypt it: not worried about this. These are NIST-standard algorithms, there built-in in most programming languages, they'll be around for a while
- Too old, trivial to crack: this is a bit more concerning to me. It's possible that some entities around the world can already crack this encryption in minutes/hours days
Regarding the second, I'm already working on an Elliptic Curve version of this.
You can check server logs to see if those URLs are ever hit.
And if you ever need to, you can abandon the original link after changing what those non-published URLs point to (something "fun", like a rickroll perhaps).
In fact, how about using the Wayback Machine to store a bunch of versions of the static page, each containing different versions of the cipher text. Only you know which date range contains the proper cipher text!
As the old adage says 'any problem can be resolved by adding one more layer of indirection'.