I want to know why I can’t filter or search events in Errors (Explore → Errors; formerly Discover) using additional / extra data or custom context.

You can’t. Extra data and custom context aren’t indexed, so they can’t be used as search filters or table columns on the Errors page.

If you need to query a value, send it as a custom tag instead. You can then filter by that tag key in Errors.

For searching both Issues and Errors, see How to search events on context or extra data?.