If everyone only starts using a package after 7d, it feels like it just means we don't find out about problematic packages until 7d later . The reason 7d works is because it is the "don't go first" effect (I assert with no evidence). But if everyone does the same delay, there's no longer a benefit to you delaying. This feels like a prisoners dilemma of no one upgrading.
I do think there is some sense in having some cool down. Automated review systems having some time to sound alarms would be good.
I'm not sure what the reporting mechanisms look like for various ecosystems. Being able to declare that there should be a hold is Serious Business, and going through with a hold or removal is a very human-costly decision to make for repo maintainers. With signficiant lag. So we are up to at least 2d, if centralized.
Ideally I'd like to see something on atprotocol, where individuals can create records on their PDS's that declare dangers. This can form a reputation system, that disincentivizes bad actors (false reports), and which can let anyone on the net quickly see incoming dangers, in a distributed fashion, real time.
(Hire me, I'll build it.)
IIUC the recent high-profile npm backdoors were mostly detected by supply-chain-security firms that ingest all package updates from the registry and look for suspicious code using automated or semi-automated analysis. Dependency cooldowns work great with this kind of thing. I agree that, if malicious packages were mostly detected via user reports, dependency cooldowns would create a prisoners' dilemma.
I don't understand what you're saying about reporting mechanisms; is there something wrong with how this is currently done?