I'd assume it should. The expand/collapse button is presented exactly the same way on deleted comments as non-deleted.
You can't collapse a massive thread if the top-level comment has been deleted. If it hasn't been deleted, you can collapse it.
Do you see it as a possible privacy issue?
I'm not sure how it could be. It is possible to collapse any other comment thread. But not for deleted comments. Not sure how allowing a user to collapse those just like any other comment would e a privacy issue.
I'll look into it in 2020 ;)
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