Well, if we are being honest, the current “devel” mailing list does not have a well defined subject. It is manageable not because it is well-organized, but because it is relatively small.
That’s why it is a bit hard to have a 1-1 map of it onto just one tag on Discourse. And I would rather recommend exploring the Discourse a bit (see Navigating Fedora Discussion — Tags, Categories, and Concepts), than just subscribing for a single thing.
But in general the engineering tag in the Project Discussion category should be a good enough starting point for all development conversations, which are not well placed into a dedicated Special Interest Group or a dedicated process, like Change Discussion. And it was in fact planned as a direct replacement of the devel@ catch-all.
Also, we can discuss this further in Using Discourse when being used to the devel mailing lists: common misunderstandings, issues, and HowTos to solve them maybe