I am not aware of this having been discussed.
From my point of view I think that ForgeFed will leverage an own vocabulary, for better coherence in it’s object model.
ForgeFed builds on Activity Pub, which builds on Activity Streams, which builds on JSON-LD, which can be processed as RDF; so the vocabulary / onthology ForgeFed will use / create can be used similar to DOAP / DOAP Bugs.
DOAP properties might even be reused directly.
Not so sure about DOAP Bugs, it seems to be condesed (from the Bug Evolution Onthology), and for ForgeFed I think we’ll rather need an broader onthology than just dbug, for example for Merge/Pull Requests.
Thank you for the hint, this made me dig a little bit deeper into OWL/RDF and the relation of JSON-LDs to that; I understand the Semantic Web is a little bit better now.