Feature summary (what you would like to be able to do and where):

Be able to provide edit summary for more (ideally all) edit actions in the Wikidata UI, like statement changes, deletions, … like in e.g. edit undo.
If not by default, then at least with some feature available in the settings.

(But the intention is not to specify custom whole edit summary, but that should be custom additional summary appended to general type of edit added automatically by default.)

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

E.g.: an user has deleted a statement for some good reasons that were not clear to other users, but was misunderstood and reverted only because no possibility to enter a summary the user wanted to share.

Benefits (why should this be implemented?):

Usual wiki feature of user being able specify a summary for the edit.
Possible misunderstandings prevention.