Sorry, that should have been "shell config command", not "documentation".
Experimental properties were hidden from the output of the shell config command, unless they were actually used, by excluding them from the DefaultConfiguration. The reason is that the semantics of "experimental" imply "optional", and "optional" implies that it could be null/unset/absent, and still work correctly. While I'm primarily concerned about prominent exposure of experimental properties through the shell and documentation, the same argument applies to anything that uses the DefaultConfiguration, because that's where the semantics of "experimental" are communicated to the user (by their omission/lack of prominence).
Because the DefaultConfiguration conveys the semantics about which properties are prominently exposed as available, I think experimental properties should continue to be omitted from it, and we add a test to ensure that the defaults for these are null and excluded. The code that depends on them being otherwise should be changed to handle them being null, or they should not be considered experimental. Alternatively, we'd have to find a better way to convey that certain properties are experimental, in spite of being so prominently exposed (and I can't think of a way to programatically communicate that, other than through omission; I think documentation that says "don't use this unless you know what you're doing; this is experimental and subject to change" is insufficient, as it will largely be ignored... if it's there, people will use it and depend on it).
The code change, for reference:
git log -n1 -p -U5 f2de427f -- core/src/main/java/org/apache/accumulo/core/conf/DefaultConfiguration.java
ACCUMULO-2460, for follow-on.