You can always try the Internet Archive/Wayback Machine - just take the URL that you're given, and prepend "http://web.archive.org/web/*/
" to it (e.g., if the original URL is "http://www.example.com/
", use "http://web.archive.org/web/*/http://www.example.com/
"); that will give you a list of occasions when the Wayback Machine took a snapshot of the URL. It doesn't always get you what you're looking for - sometimes a linked page wasn't crawled, or a robots.txt blocked access, or something like that, but it's worked for me surprisingly often.
I would, however, be wary of doing so with regards to addons for programs; if you have to resort to this, chances are that the addon is no longer supported, and there will be a significant risk that it won't work, and may even do damage. With mIRCScript addons, you can at least examine the code and see if it's doing anything that would be problematical with versions of mIRC later than it was last updated for. That does require a fairly solid knowledge of mIRCScript in both the current implementation and the version that was current at the time the addon was last updated.