Yes, it's a new feature, but one that may or may not stay depending on feedback.
If they're working well for you, please say so - it's more likely that people who are having problems will speak up.
I've gotten one report that they show up often and show very old things. :/
For me, they showed up when I enabled the mod - and then haven't since then.
So it's not looking good so far, but before I remove that mod, though I'd ask for feedback from anyone who sees this topic.
If you mean a live Facebook-isch window that appear when someone is making a post to something you replied to (I guess threads marked with subscribe or similar) then I had one yesterday. Cool feature. :)
I get a stack of black boxes along the right side every time I log in. They disappear rather quickly (maybe because I start scrolling), but seem to be the same as the contents of "Show new replies to your posts". I usually go through all the interesting boards first, and then check "Show new replies to your posts" last to see if I missed something, so those black boxes are rather useless to me so far.
I'd like to note that they actually do what they are supposed to do, too.
Perhaps it's due to old posts which the system realizes where never showed to you and are still unread? I had tons of those, just marked all as read - will tell if it helps. I would wait at least a few weeks anyway, just in case it's something silly like that that fixes itself over time.
I will definitely leave it for now, then, and hope it works out well. :)
I should note that I am still having problems with this - just now, for example, I received a notification about this (https://forum.simutrans.com/index.php?topic=8460.msg121680;topicseen#msg121680) thread, which was last updated in 2013.