There are many very old administrative RequestHandlers registered in a SolrCore that are actually JVM / node / CoreContainer level in nature. These pre-dated CoreContainer level handlers. We should (1) remove them from ImplictPlugins.json, and (2) make simplifying tweaks to them to remove that they work at the core level. For example LoggingHandler has two constructors and a non-final Watcher because it works in these two modalities. It need only have the one that takes a CoreContainer, and Watcher will then be final.
Should stay because has core-level stuff:
/admin/system – SystemInfoHandler
returns "core" level information, and also node level stuff. I propose splitting this one to a CoreInfoHandler to split the logic. Maybe a separate issue.