SocialHub has been the most prominent place for cross-project collaboration for years. This is where the FEP process was born, after all. I suspect that the need to create an account there was a major barrier to entry, but this is no longer the case as the forum supports ActivityPub.
> Cross-project forums like SocialHub can then have a dedicated category for each software they know about
There is such category already: https://socialhub.activitypub.rocks/c/software/14
It appears to be not federated, though.
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Aside:
> For the size of the group [working on federating long form articles], which as you say is not large
I wonder what they meant by this. There are lots of projects that support long form content.
