I earned two "Lifejacket" badges today, [not both on AD] on questions that had passed the criteria for its merit a long time ago.
Is it just a 'new' badge, now being applied retrospectively?
Yes, this badge and its gold version Lifeboat are new badges, announced in today's blog post Adios to Unfriendly Badges: Ahoy, Lifejacket and Lifeboat. As stated in the blog,
We’re retroactively awarding these badges so if you’ve rescued any questions in the past, that achievement will be recognized within about 24 hours. Again, this is for all sites on the network.