You'll have to check the GDF archive for the details. However, the LimeWire guys are convinced that such queries are bad. For example, it's bad for dynamic querying because searches for exact resources are less likely to succeed than keyword searches. Maybe LimeWire still supports them for TTL=1, Hops=0 but in the general case it doesn't.

The support for urn:sha1 searches in Gnutella was more or less a hack because the QRP hashcode for "urn:sha1:..." is simply added to the routing table for each file you share. This means the table will contain much more items than normally if it hashed only keywords. Therefore, there are more false-positives which means more traffic and less scalability.