The new Sourceforge team has generally done a great job. Here is a review that might help some people.
Pros:
For general project discussion, Sourceforge's traditional discussion forum is far superior to Github/GitLab issues (though I haven't tried Github Discussions beta yet). The forum can be configured for users to be able to post without creating an account (though only as a specific user named "Anonymous", not arbitrary names) which is as important feature when creating software for users who aren't likely to have Github or Sourceforge accounts.
Sourceforge download statistics tracking of releases (including graphing per country and with arbitrary timestamps) is far superior to Github, which doesn't offer even private tracking of download numbers without directly using their API. This is actually a really ridiculous situation.
Cons:
Sourceforge recently added the ability for the project administrator to mark any review as spam, which automatically hides it. This single change has completely ruined the trustworthiness of Sourceforge's reviews, as unscrupulous application authors are able to mark all poor reviews as spam so users only see good reviews. Because of this, I recommend AlternativeTo (http://alternativeto.net/), as they have better review non-interference policy.
Sourceforge's entire website seems to go into maintenance mode for a few minutes every 24 hours, which is frustrating for those in less favorable timezones.
Even after using it for a long time, Sourceforge user-interface and settings/permissions is overly complex, confusing and non-intuitive. I find Github's well designed settings page much easier. Though admittedly Github has its share of UI quirks. New Github users are understandably initially confused by the concept of Pull Requests (which should have been called Merge Requests) and the fork user-interface. As a developer familiar with both tools (and git, PRs etc) I find Github easier to use than Sourceforge, which is saying something.
Many Sourceforge projects tend to have their source code mirrored on a rarely updated Github project, which then gets forked and developed without changes being upstreamed, which causes fragmentation.
Many third-party tools (like CircleCI) tend to target only Github (and to a lesser degree GitLab/Bitbucket) and ignore Sourceforge entirely.
It's too easy for newbie users to download older releases (Github has the same issue unless you create a Github Pages site to highlight the most recent release).
Conclusion:
Sourceforge is actually a reasonable tool to develop open-source software in 2021.
For new projects I would generally suggest sticking with Github and GitLab, but for existing projects on Sourceforge changing hosting to Github may not be required.
The real killer is lack of integration of third-party tools like CircleCI. That's enough to switch to Github. But you will likely miss the excellent download statistics, anonymous support forum and user review system.
Pros:
For general project discussion, Sourceforge's traditional discussion forum is far superior to Github/GitLab issues (though I haven't tried Github Discussions beta yet). The forum can be configured for users to be able to post without creating an account (though only as a specific user named "Anonymous", not arbitrary names) which is as important feature when creating software for users who aren't likely to have Github or Sourceforge accounts.
Sourceforge download statistics tracking of releases (including graphing per country and with arbitrary timestamps) is far superior to Github, which doesn't offer even private tracking of download numbers without directly using their API. This is actually a really ridiculous situation.
Cons:
Sourceforge recently added the ability for the project administrator to mark any review as spam, which automatically hides it. This single change has completely ruined the trustworthiness of Sourceforge's reviews, as unscrupulous application authors are able to mark all poor reviews as spam so users only see good reviews. Because of this, I recommend AlternativeTo (http://alternativeto.net/), as they have better review non-interference policy.
Sourceforge's entire website seems to go into maintenance mode for a few minutes every 24 hours, which is frustrating for those in less favorable timezones.
Even after using it for a long time, Sourceforge user-interface and settings/permissions is overly complex, confusing and non-intuitive. I find Github's well designed settings page much easier. Though admittedly Github has its share of UI quirks. New Github users are understandably initially confused by the concept of Pull Requests (which should have been called Merge Requests) and the fork user-interface. As a developer familiar with both tools (and git, PRs etc) I find Github easier to use than Sourceforge, which is saying something.
Many Sourceforge projects tend to have their source code mirrored on a rarely updated Github project, which then gets forked and developed without changes being upstreamed, which causes fragmentation.
Many third-party tools (like CircleCI) tend to target only Github (and to a lesser degree GitLab/Bitbucket) and ignore Sourceforge entirely.
It's too easy for newbie users to download older releases (Github has the same issue unless you create a Github Pages site to highlight the most recent release).
Conclusion:
Sourceforge is actually a reasonable tool to develop open-source software in 2021.
For new projects I would generally suggest sticking with Github and GitLab, but for existing projects on Sourceforge changing hosting to Github may not be required.
The real killer is lack of integration of third-party tools like CircleCI. That's enough to switch to Github. But you will likely miss the excellent download statistics, anonymous support forum and user review system.