checkstyle
Document what remains at sourceforge
I'm not completely sure what is intended to remain at Sourceforge. As far as I know, the sources and the issue tracking are meant to be at Github, but the website is still hosted at Sourceforge. Do you plan to leave some parts like the website at Sourceforge, or should everything be migrated?
Ideally everything should be migrated from Sourceforge. Sources, Issues and Wiki are no brainers.
Binary downloads
Update site binaries are already located on Bitbucket for better reliability, packaged update site could go there as well
Project website
If possible move it to Github pages. The site is build on Angular 1.4 as a single page app and is dual-used to provide Eclipse help content. However, it uses a tiny amount of PHP (https://github.com/checkstyle/eclipse-cs/blob/master/net.sf.eclipsecs.doc/src/main/resources/index.php) to make the site crawlable for search engines - that won't fly on GH Pages, so an alternative is required for that.
Discussion forums
There isn't much activity, we could likely vacate the premises.
core project use google groups for questions, it is used somehow (not very frequently), but in most cases users open github issues, or go to stackoverflow.
Project website has moved to Github pages: https://checkstyle.github.io/eclipse-cs/ Redirect from the Sourceforge page has been added, works for the old update site URL as well.
There are still references to the old URL in plugin files and external sources (Bitbucket, Marketplace) which will be ironed out in the next days.
I just submitted 2 related pull requests, because I edited those files before I read your comment here. :)
Currently, no update-site is available. Marketplace (still) tries to access http://eclipse-cs.sourceforge.net/update/, which doesn't work. The github page has at its bottom net.sf.eclipsecs-updatesite/target which is complete nonsense. Where is the update-site supposed to be hosted?
Right now sourceforge itself (or at least project websites) are not available, for instance http://checkstyle.sourceforge.net/ is also not working...
It is intended that all old URLs should still work, redirecting to the new URLs. Of course when the old site is down entirely that's a problem...
The new update site URL is: https://checkstyle.github.io/eclipse-cs/update/