It is difficult to keep track of admin actions by reason, which is particularly relevant for page and revision deletions, but also for protections and blocks. We have a native drop down menu with default summaries that admins can select. Those are specified in mediawiki messages, respectively Deletereason-dropdown, Revdelete-reason-dropdown, Protect-dropdown (also used for pending changes) and Ipbreason-dropdown.
I think we should allow attaching of one tag (or maybe more than one) to each of the dropdown item, simply by editing those mediawiki pages, provided the tags are defined at Special:Tags (more on this at T20670). The tag(s) to apply may be specified after each dropdown item, separated by a special character sequence, say /*/ (we may not need to modify Xml::listDropDown, the tag may be left apparent in the menu). There should also be a way to specify an "Other reason" tag (when no drop down item is selected).
When an action is performed with a drop down item, the text of wpDeleteReasonList is split into the reason text (before /*/) and the tag(s) to apply (after /*/); there is a check that the tag(s) is(are) defined, and the action is tagged.
One issue is that the 'delete' button is often replaced via javascript with a version where wpReason is prefilled with one of those reasons, such as when there is a speedy deletion tag or AFD template. It wouldn't work for those, but it may be fixed by allowing pre-selection of a wpDeleteReasonList option in urls. The function Xml::listDropDown already contains a 'selected' parameter, so it may be filled at Article.php with a url param.