Seen this exact thing. The portal becomes a single point of friction instead of solving for the actual workflow. Developers still dig through Slack history and old PRs because that's where the real answers live.
Best outcome I've seen: minimal portal that's basically a router. Links to actual sources of truth - the mono repo README, the service's own docs, the runbook. Curate ruthlessly, but don't duplicate. Devs learn fast where to actually look, and you're not maintaining two versions of everything.