This is actually fairly simple and well understood: leave instructions in your will.

"Notify <Provider> to delete my account" is a perfectly valid instruction to leave for an executor.

You could leave behind a password cache with a master password left in your will, but I suspect much of this still runs on trust. I'd imagine (I haven't tried), that "X has died, please take action Y" is a fairly reliable social engineering vector if you have a convincing "proof" that X has died.

It's worth noting that the executor isn't hard forced to carry out your wishes, the legal recourse for them not doing so comes from other beneficiaries ability to take legal action against the executor. If those other beneficiaries don't care much for enforcement, then you might prefer technical methods such as the submission.