I agree. Everyone who's posted here should submit the bug report using this link: Feedback - Pages - Apple. It's the only way they're really going to do anything about it—unless they happen to have a regression test that somehow surfaces between now and the next release.
If they did have said test, there's a good chance they wouldn't have made this change.
I find this particular feature really annoying because some apps do SHIFT+ENTER, some do CTRL+ENTER and some do both. It's even worse because the current Mac version of Excel requires OPTION+ENTER to get soft returns in a cell!
But the principle here is that you've trained your users for decades how to do this—and then you change it. There's no reason they couldn't have just added an additional shortcut, because I've never heard of SHIFT+ENTER having any other function than this.
SUBMIT YOUR FEEDBACK!
It's the only way to effectively b*tch about stuff like this. Comments here aren't really going to do much.