That’s a double indirection. So you want to have the FEP url de-referencable from the @context. It points to this Discourse forum, which is a manual (and editable afterwards, but by one single member unless it is a wiki post) copy of a markdown in a git repository on Codeberg. And that url points to the top of the main branch, not a particular revision (commit), so it might change after you included it. If you take for instance the W3C specs themself, they point to the revision:
This version:
Latest published version:
Right now we have an open issue on the FEP process on how to change or extend them once they became FINAL. It may impact how to reference ‘This version’. Also we talked about stable domains to use and w3id.org and purl.org were mentioned.
Maybe the ‘This version’ would be something that includes the short-form SHA1 hash of the commit that finalized it, combined with the stable URL.
https://w3id.org/fediverse/fep-400e/publicly-appendable-activitypub-collections/eb893ae1a5
In that example the fep-400e is taken as FEP identifier and the title separate (similar to Discourse URL’s where you can omit the title), so the shortest-hand form of the ‘Latest version’ would be:
https://w3id.org/fediverse/fep-400e/