I have the same concern. Old threads being posted on is an issue for a few reasons, but the least of those reasons (perhaps JMO) was that they were brought to the attention of everyone else again (despite how grumpy it made some 🤣 ). 

If old threads are not locked and can still be posted on, I struggle to see the logic behind making them effectively invisible -  no one is going to look beyond the first page for new posts, and only threads intended for continual input or of special interest are going to be bookmarked or floated by individual users.

The whole point of forums like these is supposed to be so that people who need help, can get help - they're meant to be specifically geared towards creating a low(er)-cost community that uses volunteers to reduce the burden on actual CS, which in turn resolves issues for members so that they can continue to use the site, and keeps people engaged with eBay. 

So, if it's an intentional change, it's a very poorly informed one, IMHO.