"they will be increasingly infrequent as Leopard becomes increasingly antiquated." The two "facts" have basis: fewer updates over time, and Leopard is getting older ("antiquated"? Like a Ming vase, it should increase in value as it becomes an antique). The decrease in updates is caused by not needing more updates. Apple does not slowly turn their backs on older OS versions. They offer any and all useful updates until some date when all update support ceases. That date is some time ahead for Leopard.
I have one computer running OS 10.5.8, and there have been no updates in the past two months. I just ran Software Update and still nothing new. Maybe next month there will be one or two updates.
The types of updates Apple releases and that are fed to the Software Update app are...
-- OS updates (for the OS itself, as well as for QuickTime, iTunes and other integrated apps). These only come when Apple develops some important improvement for those elements.
-- iLife updates. These only come when Apple develops some important improvement for an iLife app.
-- Pro app updates (Final Cut Pro, Logic Pro, etc.).
-- Java updates. These are often not needed or not possible for older versions of Java.
-- Security updates. These are related to vulnerabilities discovered in the UNIX or OS X system, as well as profiles to detect and remove well-documented Trojan horse apps (malware designed to appear like something good).