FWIW It’s only the intro topics that get an auto-close as replying to those 12 years later didn’t seem useful. 
Sorry for the big burst of Unreads that gave, but that should be a one off and you can clear those out with the ‘Dismiss’ button if they’re bugging you (I added some screenshots to this topic What are you currently doing? - #4813 by Ian_Chimp if that helps).
Older topics aren’t closed en masse, but when ones pick up a reply when there’s already a more recent version it makes sense to slide the posts over to the current one so as not to splinter the conversation and have two of the same running at the same time (as it avoids confusion, and people cross-posting/duplicating their answers in both). Though this isn’t a new thing, and you’ve probably seen that happen a fair few times already.
As @RacyRosalee said, closed just means no new replies. If you search for something and there’s no open topic on it then starting a new topic would be the best thing to do. 
Same for if all the open topics you find are a decade old and you want a fresh perspective. I think the old topics are great for reading through and seeing what advice was given in similar situations over the years, but not all of them will be the perfect fit to bump to the top of the Latest list with a new reply.
Personally, I’m loving the Related Topics section. I’m finding it a fantastic little rabbit hole of linked info that I keep getting lost down. 
