This archive is a project started by Lord Matt to ensure that at least most plugins (and skins) are stored in such a way that even if the source vanashes it is still available to the community.
As it is at sourceforge.net it means that the archive versions should be available on about 12 to 18 university servers the world over. (now that can't be bad).
You can find the summary for the page here.
At the moment the plan is to achive just one copy of each plugin. However if plugin authors want to join in and maintain an up to date copy it is clear that no one will complain.
The next step is to set up CVS and open it up for collaberative creation of sets of or complex and advanced plugins.
That's a great idea, Matt. This CVS archive is a good backup, if well maintained. There was/is another project for this, too: http://sourceforge.net/projects/nucleusplugs/
I'm actually working on something similar for plugins with nucleus-plugins.org That site will have a SVN repository for plugin authors. And a Trac bug reporting, repository browsing system.
It seems that great minds think alike. -Matt