@experimental decorator to mark "yet unstable API classes and methods that can be deprecated without notice"

[yashaka]

Consider implementing something like: https://github.com/vilic/deprecated-decorator
called as "@experimental"
when it's applied to a method or class
and this method or class is used by the end user
and some option like stage.warnExperimental = true (false by default),
then corresponding warnings should be logged
for example something like: the xyz method is experimental and may be deprecated and removed without notice