iOS 26.4 23E246 is the only signed IPSW for the iPhone 12, so no downgrade is possible.
To kind of answer the other part of your question: the EU now requires at least 5 years of smartphone OS updates. That was up in October 2025, but I guess Apple wanted to put out one more update so they wouldn't get in trouble for cutting it off in September? Not sure they care about EU fines anymore, but someone at Apple made the decision to release it. In fact, iOS 27 this year MAY still be available for the 12, because even though the 11, 12, and 13 are all going to be 5 years or older, the SE3 with the same 4GB of RAM is only going to be 4 years old, so should continue to get updates. I don't recommend it though. These OSes were designed for new phones with 8-12GB RAM. It reminds me of Horrible Bosses: you can drag race in a Prius, but you won't win much.