Setting pluginData on closed cards from other cards/REST api

Our Power-Up establishes card dependencies and from a parent card we want to read and set certain attributes on child cards, which might already have been archived/closed. Using t.set(cardId, …) usually fails if the card with ID “cardId” is closed and was not previously shown/opened for some reason.

Is there a way to reliably set/read pluginData from closed cards from a third card or a backend, either via the Client Library or the REST api?

There is no way to PUT/POST/DELETE any pluginData via the REST API. However, you can make a GET request for it, even on archived cards.

There is a brief mention of API access here: https://developers.trello.com/docs/getting-and-setting-custom-data#section-rest-api-access (more documentation on this is in the works). And the pluginData card route is documented here: https://developers.trello.com/reference#cardsidplugindata

Good to have this clear, Is setting and deleting pluginData using the API on the roadmap?

Nope. Unfortunately, not on the roadmap.