4.3.2.1 Single Connection
The minimum block of IP address space assigned by ARIN to end- users is a /20. [...]
4.3.2.2 Multihomed Connection
For end-users who demonstrate an intent to announce the requested space in a multihomed fashion, the minimum block of IP address space assigned is a /22. [...]
4.4 Micro-allocation
ARIN will make micro-allocations to critical infrastructure providers of the Internet, including public exchange points, core DNS service providers (e.g. ICANN-sanctioned root, gTLD and ccTLD operators) as well as the RIRs and IANA. These allocations will be no longer than a /24 using IPv4 or a /48 using IPv6. [...]
As far as I know, all of the PI /24's are thus "legacy" in nature.
As the above snippet from the policy manual suggests (and as my experience confirms) there are recent assignments made to end users by ARIN under the micro-allocation policy which were made with the expectation that individual /24s would be advertised globally. Clearly these are not the most usual case, as the description of those who qualify for such assignments above indicates, but it would be a mistake to assume that *all* /24 assignments are legacy.
Actually, generally, the expectation under 4.4 is that the addresses will not be advertised at all for the most part, since, generally, there's no need to advertise the route to the exchange point, itself, into the global routing table. 4.4 is intended to support internet exchanges, ala MAEs, etc. In terms of 4.3.2.1 and 4.3.2.2, I believe ARIN has worked very hard to express no expectation or intent about how assignments relate to route advertisements and routing policy. Owen