I don't understand. I just tested this, and after installing maven-projecthelp-plugin - with the plugin registry enabled - the new projecthelp entry was in fact recorded.
As for -cpl, it should be working again. Without it, there's no way it0013 and it0020 will work as-is...they will require some sort of hackery with <defaultGoal/> and a multi-module build in order to make this feature testable from the it0000 series.
What exactly is it you want me to do here? Checking LATEST will result in normal users potentially getting snapshot plugin versions (ie. unstable), so I don't think that's a great way to go. On the other hand, some people (namely plugin devs) will want to be able to use the snapshot versions of plugins, so sticking with RELEASE doesn't really work either. I'm not sure what would happen if we allowed a locally installed -SNAPSHOT version to persist (ie. will it resolve the snapshot updates on the remote repo?), but one thing is sure about that: the developer would have to run an install on that plugin at least once, in order to get the entry into the plugin registry (or hand-edit).
All of the above seems pretty nuanced and more than a little unintuitive to me. I think, since -cpl is currently working and NOT the default, we should just leave it alone. We have bigger fish to fry when it comes to plugin usage, IMO.
For example, executing a plugin within the same build in which it was installed/built (esp. when that build is a reactored build, which it is likely to be).