They did it because people were spam pinging groups and it's annoying, I think.
You can still ping lists of people if you want.
We haven't been able to ping forum groups in advertising forums for a long time. I believe they disabled it because users requested for it to be changed due to abuse.
- It's been like that, I don't know the reason why, but it was changed so that you can't use them pretty soon after they introduced the group tags.