Are you sure? I just played a game against someone who played Jailer. It wouldnt let me target any of his minions with Power Word Death or some other spells either
It's in the unannounced changes here: https://www.hearthpwn.com/news/9875-patch-25-6-unannounced-changes-data-mining
Scroll down to Strings.
I haven't come across anyone playing the Jailer or other Immune minions lately so haven't tried it myself either.
It's a change in tooltip, not a change in functionality. I'm pretty sure this has happened before, with the tooltip change being reverted shortly after (it was a mistake).