I think I saw your patch for Issue 18814 proposes to use UnicodeTranslateError. Is there any other case where it is used, either currently or in the past? All I know of it is the documentation, which says it is raised “during translating”.

Experimenting with the constructor reveals that the “object” attribute is only allowed to be a text string (not bytes). So perhaps “translating” actually means converting from text strings to text strings, like “rot-13”. It would be nice if this were documented somewhere, rather than just saying translating is now supported.