Yes, I don't like this loss of functionality, either.
Tracking such things is simple to the point where I'm not sure it's worth designing a grand generalized systemwide means of doing it. At least if it means that we don't enhance forums for months and months while waiting for someone to invent such Grand Unifying Functionality.
There's also the *real* "new" semantics I added to SSV1 (ACES) which wasn't reimplemented in forums, which kinda surprised me given that Sloan felt it important enough to pay me to do it for ACES two summers ago. In other words in ACES the forums package flags as "new" those threads that have posts you actually haven't read, rather than threads with posts less than a week old. Phil's bboard's "new threads" was an "n-days-old" display, too, I greatly prefer the smarter "threads with posts I haven't read" display.
Of course there's expense in such bookkeeping so we'd want to make it possible to turn off for those who don't want to pay the price but it would be nice to have.
