As discussed in the concurrent Google Search thread, and in the Webmaster help page for "Soft 404 Errors," this "301 all missing pages to one page" is a very bad idea.
Better to serve a custom 404 or 410 error page for those URLs, and to make the "archive" page the ErrorDocument for the directory to which you rewrite them.
For example, in the "forum" directory .htaccess file:
RewriteCond %{QUERY_STRING} ^1234$
RewriteRule ^old-URL$ /archive/file-path-which-does-not-exist.hmtl [L]
and in /archive/.htaccess
ErrorDocument 404 /archive/archived-stuff-page.html
-or-
Forum directory .htaccess file:
RewriteCond %{QUERY_STRING} ^1234$
RewriteRule ^old-URL$ /archive/forum-removed.html [L]
and in /archive/.htaccess
RewriteRule ^forum-removed\.html$ - [G]
#
ErrorDocument 410 /archive/archived-stuff-page.html
Note that neither /archive/file-path-which-does-not-exist.hmtl nor /archive/forum-removed.html exist as real files.
In either the 404-Not Found or the 410-Gone implementation example, the archived-stuff-page.html should contain links to your home page, major categories, HTML site map, and site search facility. It can even look almost exactly like your home page or old "forum" page if you like. As long as a 404-Not Found or 410-Gone status is returned to the client, you won't have any trouble with search indexing or ranking.
The above examples may actually be more complicated that what you really need. If your old forum URLs all indicated a subdirectory, then you wont need the rewrite; Just put the errordocument directive and the custom error page in that previously-existing subdirectory.
Jim