What purpose would that serve apart from satisfying idle curiosity? The reasons why an ASN is not advertised are of concern only to the operator of that ASN.
It depends. Is it resposibility of RIR to monitor usage of resources and search for unused ones? If so, there should be some way to tell RIR "This ASN is in use even if you can't see it, it is OK, don't bother me!". From my point of view it is better to track changes of announced -> not-announced and vice versa (when stated that they should not be), then list of all ASN which are not announced. Just because it doesn't carry an information if it is an intentional or not. Turning off such flag automatically would then work as fail-safe for abusing such flag for usual cases when ASN is announced but you would not want RIPE NCC to check on you. If you don't think that RIR should not track usage or resources in its region, then you are right, it would be not needed curiosity.
I think this idea has merit. a page that lists "lost" ASNs. Possible downside is that some randomer could claim to "own" it but it may be possible for the NCC to establish whether they have a right to it.
If you make just a list, apart of the downside you have mentioned, there is possibility that your ASN will get to such list periodically. How would you indicate that you ASN is not announced publicly for a reason? If you are planning making such list in the waves over and over again, it would be fine. Best Regards Martin Hunek