EX: if I read a post/comments in my web browser and then pull up lemmy on my mobile app it will show as read there as well.
This used to work on reddit where I was able to browse posts on Old.Reddit during the day and in the evening when I opened RedditSync they would show as read as well, but it required purchasing reddit premium.
I know Voyager does, at least on iOS. I would suspect most of them do.