I took a look at the video game devlogs. So here's what I would say: posting a thread asking for design feedback (with specific questions) or asking for help brainstorming, or putting forth a mechanic you're really proud of and want to discuss, etc. would definitely fit in the design forums. As the project progresses or the person implements the feedback, it seems to make sense to continue to ask questions and share in the same thread for that project.
However, a lot of the devlogs I looked at in video games were "here's my project and I'm posting updates as I go" which seems like a better fit for the devlog function built into itch.io or fell very much in the "self-promo" side of things.
I'd say it comes down to intent: is the thread seeking to have a conversation around the design, or seeking to show it off? Conversation and feedback is awesome; showing off a project is better placed in the "Share Your Projects" stickies.
Does that make sense?