ActivationUnMatchListener is part of kie-internal, so its not public API.

ActivationUnMatchListener is only used by OptaPlanner (probably):
jBPM and downstream drools module don't use it:
https://github.com/search?q=org%3Akiegroup+ActivationUnMatchListener&type=Code
Community users probably don't use it either, because no one mentions it on StackOverflow:
https://www.google.be/search?q=%2BActivationUnMatchListener+site:stackoverflow.com&*

The current behaviour of ActivationUnMatchListener is a pain because it doesn't unmatch if a RHS fires again, causing an imbalance. OptaPlanner works around this through a hack (which fails for PLANNER-761).

For example, given this rule

global int score = 0;
rule R
when
    Wine(age < 10, $age : age)
then
    score += $age;
    addUnmatchListener(() -> score -= $age);
end

This works for a normal match and unmatch event:

Wine w = new Wine(3);
insert(w);
fireAllRules(); => score = 7;

w.age = 50;
update(w);
fireAllRules(); => unmatch triggers => score = 0; // GOOD

So events are like this:

RHS
unmatch

However if we have a rematch:

Wine w = new Wine(3);
insert(w);
fireAllRules(); => score = 7;

w.age = 4;
update(w);
fireAllRules(); => no unmatch triggers => score = 13; // BAD Should be 6. Unmatch should have triggered.

w.age = 50;
update(w);
fireAllRules(); =>  unmatch triggers => score = 7; // Should be 0.

Because the events are unbalanced:

RHS
RHS
unmatch

I'd argue that anyone using the ActivationUnMatchListener would want to the have balanced events.