Thanks for the questions Gijs. What was meant by "don't include private windows" was not having closed tabs from private windows appear in the recently closed tabs for all windows list. So it should exclude private windows and keep private tabs local to each private window.
Bug 1819675 Comment 14 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Thanks for the questions Gijs. What was meant by "don't include private windows" was not having closed tabs from private windows appear in the recently closed tabs for all windows list. So it should exclude private windows and keep private tabs local to each private window. I've updated the description.