With v0.21.0 TagMultiselectWidget has been integrated into OOUI and since then, it has been proven its stability while being used successfully in prominent places like RCFilters and AdvancedSearch.
As a consequence, let's replace, deprecate and finally remove inferior CapsuleMultiselectWidget from OOUI. It would also improve situation for newcomers who need to decide what they are going for and having presented two mostly similar in basic functionality widgets.
 Replace (current known usage):
 core HTMLForm capsulesWidget in multiselect.js
 core CategoryMultiselectWidget (and dependent CategoryCapsuleItemWidget) in mw.widgets.CategoryMultiselectWidget.js
 Special:APISandbox in mediawiki.special.apisandbox.js
 Deprecate in OOUI