Should angular $watch be removed when scope destroyed?

No, you don't need to remove $$watchers, since they will effectively get removed once the scope is destroyed.

From Angular's source code (v1.2.21), Scope's $destroy method:

$destroy: function() {
    ...
    if (parent.$$childHead == this) parent.$$childHead = this.$$nextSibling;
    if (parent.$$childTail == this) parent.$$childTail = this.$$prevSibling;
    if (this.$$prevSibling) this.$$prevSibling.$$nextSibling = this.$$nextSibling;
    if (this.$$nextSibling) this.$$nextSibling.$$prevSibling = this.$$prevSibling;
    ...
    this.$$watchers = this.$$asyncQueue = this.$$postDigestQueue = [];
    ...

So, the $$watchers array is emptied (and the scope is removed from the scope hierarchy).

Removing the watcher from the array is all the unregister function does anyway:

$watch: function(watchExp, listener, objectEquality) {
    ...
    return function deregisterWatch() {
        arrayRemove(array, watcher);
        lastDirtyWatch = null;
    };
}

So, there is no point in unregistering the $$watchers "manually".

You should still unregister event listeners though (as you correctly mention in your post) !

NOTE: You only need to unregister listeners registered on other scopes. There is no need to unregister listeners registered on the scope that is being destroyed.
E.g.:

// You MUST unregister these
$rootScope.$on(...);
$scope.$parent.$on(...);

// You DON'T HAVE to unregister this
$scope.$on(...)

_{(Thx to @John for pointing it out)}

Also, make sure you unregister any event listeners from elements that outlive the scope being destroyed. E.g. if you have a directive register a listener on the parent node or on <body>, then you must unregister them too.
_{Again, you don't have to remove a listener registered on the element being destroyed.}

Kind of unrelated to the original question, but now there is also a $destroyed event dispatched on the element being destroyed, so you can hook into that as well (if it's appropriate for your usecase):

link: function postLink(scope, elem) {
  doStuff();
  elem.on('$destroy', cleanUp);
}

I would like to add too @gkalpak's answer as it lead me in the right direction..

The application I was working on created a memory leak by replacing directives whom had watches. The directives were replaced using jQuery and then complied.

To fix i added the following link function

link: function (scope, elem, attrs) {
    elem.on('$destroy', function () {
        scope.$destroy();
    });
}

it uses the element destroy event to in turn destroy the scope.

Should angular $watch be removed when scope destroyed?

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